Minnesota: Effects of Rain Gardens on Water Quality

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Effects of Rain Gardens on Water Quality

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Minnesota: Effects of Rain Gardens on Water Quality

  1. 1. Effects of Rain Gardens on the Quality of Water inthe Minneapolis–St. Paul Metropolitan Area ofMinnesota, 2002-04Scientific Investigations Report 2005-5189Prepared in cooperation with the Metropolitan Council of the Twin CitiesU.S. Department of the InteriorU.S. Geological Survey
  2. 2. Cover: Photograph showing rain-garden site at Lakeville, Minnesota, September 2004.(All photographs in this report were taken by U.S. Geological Survey employees.)
  3. 3. Effects of Rain Gardens on the Quality ofWater in the Minneapolis–St. PaulMetropolitan Area of Minnesota, 2002-04By Lan H. TornesScientific Investigations Report 2005-5189Prepared in cooperation with the Metropolitan Council of the Twin CitiesU.S. Department of the InteriorU.S. Geological Survey
  4. 4. U.S. Department of the InteriorGale A. Norton, SecretaryU.S. Geological SurveyP. Patrick Leahy, Acting DirectorUse of trade, product, or firm names in this report is for identification purposes only and does not constituteendorsement by the U.S. Geological Survey.Mounds View, Minnesota, 2005For additional information write to:U.S. Geological SurveyDirector, USGS Water Science Center of Minnesota2280 Woodale DriveMounds View, MN 55112http://mn.water.usgs.gov/For more information about the USGS and its products:Telephone: 1-888-ASK-USGSWorld Wide Web: http://www.usgs.gov/Although this report is in the public domain, permission must be secured from the individual copyright ownersto reproduce any copyrighted materials contained within this report.Scientific Investigations Report 2005-5189
  5. 5. iiiContents Abstract ....................................................................................................................................................................... 1 Introduction................................................................................................................................................................. 1 Purpose and Scope ........................................................................................................................................ 2 Acknowledgments .......................................................................................................................................... 3 Description of Study Sites ............................................................................................................................. 3 Methods .......................................................................................................................................................... 10 Water Quality at Rain-Garden Sites...................................................................................................................... 11 Chanhassen ................................................................................................................................................... 12 Hugo ................................................................................................................................................................ 12 Lakeville .......................................................................................................................................................... 13 Minnetonka .................................................................................................................................................... 13 Woodbury ....................................................................................................................................................... 15 Effects of Rain Gardens on Water Quality ........................................................................................................... 15 Implications of Results ............................................................................................................................................ 18 Summary.................................................................................................................................................................... 21 Selected References ............................................................................................................................................... 22FiguresFigure 1. Schematic diagram of expected processes and monitoring points of a rain garden. .................... 2Figure 2. Location of rain-garden sampling sites and percentage of clay in soils in the Minneapolis- St. Paul metropolitan area of Minnesota................................................................................................. 4Figure 3. Rain-garden configuration at Chanhassen, Minnesota. ....................................................................... 6Figure 4. Rain-garden configuration at Hugo, Minnesota. .................................................................................. 8Figure 5. Rain-garden configuration at Lakeville, Minnesota. ............................................................................ 9Figure 6. Rain-garden configuration at Minnetonka, Minnesota....................................................................... 10Figure 7. Rain-garden configuration at Woodbury, Minnesota. ........................................................................ 11Figure 8. Distribution of specific conductance and phosphorus, chloride, and suspended-solids concentration at each of the five rain-garden sites sampled in the Minneapolis-St. Paul metropolitan area of Minnesota, 2004-04. ............................................................................................. 16Figure 9. Distribution of specific conductance of water samples collected from inflow and rain- garden lysimeter and well at each of the five sites sampled in the Minneapolis-St. Paul metropolitan area of Minnesota, 2002-04. ............................................................................................. 19Figure 10. Change in chloride, nitrite plus nitrate nitrogen filtered, and total phosphorus concentration at the Hugo and Woodbury rain-garden sites in the Minneapolis-St. Paul metropolitan area of Minnesota, 2002-04...................................................................................................................... 20
  6. 6. iv Tables Table 1. Rain-garden sites sampled in the Minneapolis - St. Paul metropolitan area of Minnesota ........... 5 Table 2. Lithologic log of wells installed at rain-garden sites in the Minneapolis - St. Paul metropolitan area of Minnesota ............................................................................................................... 7 Table 3. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Chanhassen, Minnesota, 2002-04 .......................... 12 Table 4. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Hugo, Minnesota, 2002-04 ....................................... 13 Table 5. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Lakeville, Minnesota, 2002-04 ................................ 14 Table 6. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Minnetonka, Minnesota, 2002-04 ........................... 14 Table 7. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Woodbury, Minnesota, 2002-04 .............................. 15 Table 8. Median specific-conductance value and chloride concentration at each of the five rain-garden sites in the Minneapolis-St. Paul metropolitan area of Minnesota, 2004 ................. 18 Conversion Factors, Datums, and Abbreviated Water-Quality Units Multiply By To obtain inch (in.) 2.54 centimeter (cm) foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) square mile (mi2) 2.590 square kilometer (km2) Water temperature is reported in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by using the following equation: °F=(1.8×°C)+32. Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27). Chemical concentration is reported only in metric units. Chemical concentration is reported in milligrams per liter (mg/L), which is a unit expressing the mass of solute (milligrams) per unit volume (liter) of water. For concentrations less than 7,000 milligrams per liter, the numerical value is about the same as for concentrations in parts per million. Specific conductance is reported in microsiemens per centimeter at 25 degrees Celsius (µS/cm).
  7. 7. Effects of Rain Gardens on the Quality of Water in theMinneapolis–St. Paul Metropolitan Area of Minnesota,2002-04By Lan H. TornesABSTRACT InTRoDUCTIon Rain gardens are a popular method of managing runoff Several means have been used to deal with storm-waterwhile attempting to provide aesthetic and environmental ben- runoff in urban areas. Traditional systems of curbs, gutters,efits. Five rain-garden sites in the Minneapolis – Saint Paul and storm drains carry storm-water runoff directly to localmetropolitan area of Minnesota were instrumented to evaluate streams and rivers without any bioretention, filtering, process-the effects of this water-management system on surface and ing, or attenuation of runoff. This direct runoff can result insubsurface water quality. Most of these sites were in suburban erosion and delivery of sediment and nutrients to receivinglocations and frequently in newer developments. Because waters. Catch basins allow sediment to settle and retain nutri-of this they were affected by changing hydrology during the ents that reduce the amount of material transported to nearbycourse of this study. streams and lakes. Although effective at attenuating runoff Less-than-normal precipitation during much of the study and suspended solids, they often create a hazard and can createmay have resulted in samples that may not be representative of breeding grounds for mosquitoes and other pests.normal conditions. However, the resulting data indicate that Rain gardens are becoming important landscape tools forproperly designed rain gardens enhance infiltration and can water managers and land-use planners. These retention basinsreduce concentrations of dissolved ions relative to background provide water storage and an area for infiltration of storm-conditions. water runoff while providing attractive landscaping. Rain The runoff events in one rain garden and several runoff gardens are designed to retain runoff and encourage infiltrationevents in the other rain gardens produced no sampled over- to ground water. Retention encourages uptake and biodegra-flow during this study because the gardens captured all of dation of compounds that may be present in the runoff. Thethe inflow, which subsequently infiltrated beneath the land assumption of rain-garden design is that sediment, nutrient,surface, evaporated, or transpired through garden vegetation. and other chemical removal occurs as the runoff comes inWhere measured, overflow had reduced concentrations of contact with the soil, bacteria, and roots of shrubs or othersuspended solids and most nutrient species associated with vegetation within the rain garden. It also is assumed that thisparticulate material, as compared to inflow. Many of these process results in improved surface-water overflow quality,materials settle to the bottom of the rain garden, and some and improved quality and amount of ground water as a resultnutrients may be assimilated by the plant community. of infiltration. Site design, including capacity relative to drainage area Rain gardens are being installed around the United Statesand soil permeability, is an important consideration in the (Rain Garden Network, 2005), including in several communi-efficiency of rain-garden operation. Vegetation type likely ties around the Minneapolis-St. Paul metropolitan area of Min-affects the infiltration capacity, nutrient uptake, and evapo- nesota. Although data have been collected from some sites,transpiration of a rain garden and probably the resulting water few published studies document effects of the rain gardens onquality. The long-term efficiency of rain gardens is difficult to the quality of surface and ground water. To help address thisdetermine from the results of this study because most are still need for information, a study was done by the U.S. Geologicalevolving and maturing in relation to their hydrologic, biologic, Survey (USGS) in cooperation with the Metropolitan Counciland chemical setting. Many resource managers have ques- of the Twin Cities, Department of Environmental Services,tioned what long-term maintenance will be needed to keep during 2002-04. The objective of the study was to describerain gardens operating effectively. Additional or continued the quality of water as it flowed into and through rain gardensstudies could address many of these concerns. following runoff events. The expected processes that occur in rain gardens and an idealized schematic diagram of typical site instrumentation are shown in figure 1. Runoff water is directed into the rain
  8. 8. 2 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04Figure 1. Schematic diagram of expected processes and monitoring points of a rain garden.garden through storm drainage and subsequently allowed to rus, chloride, and gross measures of dissolved constituents.pond temporarily until water can infiltrate into the ground and Although the changes in mass transported throughout the(or) be taken up by the garden vegetation. Rain gardens are system relative to sources were not measured, the data providedesigned to overflow during large runoff events with a speci- initial information to evaluate measured concentrations infied recurrence interval. Retained water undergoes a variety components of the water system in a rain garden that wereof biotic or abiotic processes that include uptake by vegetation sampled and determine how they interrelate at each of thewith subsequent transpiration, evaporation, and infiltration. sites sampled. Other important factors including precipita-Sedimentation and biological transformation also remove tion characteristics, antecedent conditions, and flow volumessuspended solids, nutrients, and other contaminants that could from which to compute loads were not measured. Long-be detrimental to receiving waters. duration storms that exceeded the capacity of the automatic samplers were not adequately sampled. Other factors that could contribute to a better understanding of the systemsPurpose and Scope including delineation of drainage areas, contributing drainage areas, and detailed information on land-use characteristics also This report discusses the results from sampling rain-gar- were beyond the scope of this study. Determining how theseden sites located throughout the Minneapolis-St. Paul met- samples relate to the existing climate or changing climate,ropolitan area during June 2002 through October 2004. The changes in land use, and other factors also was beyond theamount of precipitation, the volume of inflow and overflow, scope of this report.and the amount of infiltration and other losses were not mea- Results in this report will improve understanding aboutsured and were not considered in this report. Sources of water the fate of chemical constituents transported to rain gardensto the rain gardens were evaluated only qualitatively because in runoff and can be used by water managers and land-usemost sites had relatively flat contributing areas that may have planners to better understand the environmental impacts andsporadically added to the inflow. effectiveness of rain gardens in protecting water quality. This Sampling focused primarily on determining the concen- can lead to improved designs and enhanced protection fortration of a few selected constituents considered to be indica- surface and ground water. Findings about the significance oftive of runoff including suspended solids (particulate material local site conditions, such as soil texture and permeability, willcarried in suspension by flowing water, also called residue, allow for improved rain-garden design that more effectivelytotal at 105 degrees Celsius, suspended), nitrogen, phospho-
  9. 9. Introduction 3treats storm-water runoff. Results should be transferable to The sampling sites at each of the rain gardens sampledother areas of the nation. for this study are listed in table 1. Also included is informa- tion, such as site identifier, that would be useful in locating additional information about these sites and the actual dataAcknowledgments that are currently (2005) available. The climate across the area ranges from relatively warmer The USGS gratefully acknowledges the Metropolitan in the southwest to relatively cooler in the northeast. How-Council of the Twin Cities, Department of Environmental ever, these long-term climate patterns often are confounded byServices, who supported and helped design this study. Grati- weather systems that have local effects.tude also is extended to all of the local units of government, Climate conditions in the area are relatively uniform.developers, and landowners who assisted with installation Normal mean monthly temperatures at the Minneapolis -St.and allowed continued access to these sites. The following Paul International Airport (1971-2000) vary from 70.6°Fcompanies provided maps of rain-garden design that were (August) to 13.1°F (January) and average 45.4°F annually.used to sketch simplified diagrams of sites shown in figures 3, These normal means are slightly higher in the south and west4, 5, and 7, respectively: Bonestroo, Rosene, Anderlick, and and lower in the north and east. Inner-city areas are slightlyAssociates; Emmons and Olivier Resources, Inc.; Westwood warmer than outlying areas at all times of the year (MinnesotaProfessional Resources; and Barr Engineering. Department of Natural Resources, 2005a). Seasonal variability in precipitation also occurs in theDescription of Study Sites area. Nearly two-thirds of normal mean annual precipitation falls during the growing season from May through September The Minneapolis-St. Paul metropolitan area of Minnesota and only 8 percent of the normal mean annual precipitationis situated on relatively flat to gently rolling land that was falls in the winter (December through February). Normalmostly prairie before settlement. Relief is much greater near annual precipitation varies across the area from about 29 tothe rivers and streams that cross the area. Most of the area has 30 in. and increases from the southwest to the northeast. Thean altitude of about 1,000 ft or less. The rain-garden sites that lowest normal mean monthly precipitation occurs in Augustwere studied range in altitude from about 925 to more than (Minnesota Department of Natural Resources, 2005b).1,000 ft. During parts of this study, precipitation was less than nor- Five rain-garden sampling sites were selected with mally would be expected. This resulted in collection of fewerthe input from the Metropolitan Council of Environmental samples than were anticipated during certain seasons.Services and other interested stakeholders. The sites repre- Climatic variations are minor among the rain-gardensent a wide range of hydrologic and land-use conditions that sites. Weather variations resulting from convective thunder-also represent a range of impervious surface conditions that storms are more likely to create variability among sites andincluded parking lots, driveways, walkways, and roofs. The result in substantial differences in the amount of precipitationrain gardens also may receive runoff from grassy areas includ- delivered to the rain gardens studied. Use of other hydrologicing athletic fields and lawns. data, including records of precipitation, inflow and overflow The five sites (fig. 2) are located in the communities of volumes, and rates of evapotranspiration, which could haveChanhassen, Hugo, Lakeville, Minnetonka, and Woodbury. been useful to estimate loading and attenuation of materialsThe sites are distributed across an area of nearly 4,000 mi2 delivered to the rain gardens, was beyond the scope of thiswithin the seven-county Minneapolis–St. Paul metropolitan study.area. The percentage of clay in the uppermost 5 ft of the soil The effectiveness of rain gardens is related to the topo-profile (fig. 2) was estimated from the State Soil Geographic graphic and land-use settings of each site, including the texture(STATSGO) data for Minnesota by averaging the percentage and hydraulic conductivity of soils and unconsolidated glacialof clay in mapped units of the soil profile. The STATSGO data deposits that underlie the sites. Most of the rain-garden sitesset is “… a digital general soil association map developed by were chosen with the intention of capturing runoff imme-the National Cooperative Soil Survey. It consists of a broad diately downstream from an impervious surface, such as abased inventory of soils areas that occur in a repeatable pattern roof or parking lot, and soil characteristics were not alwayson the landscape and that can be cartographically shown at as important a consideration as location. Although the soilthe scale mapped. The soil maps for STATSGO are compiled types (fig. 2) are a general guide, local variations may occurby generalizing more detailed soil survey maps. Where more that cannot be mapped at the scale shown. Soil tests and otherdetailed soil survey maps are not available, data on geology, engineering considerations similar to a percolation test taketopography, vegetation, and climate are assembled, together into account the variables that contribute to infiltration in awith Land Remote Sensing Satellite (LANDSAT) images. rain garden. The site at Lakeville apparently had a highlySoils of like areas are studied, and the probable classification permeable substrate that resulted in little or no overflow fromand extent of the soils are determined.” (U.S. Department of the storms sampled, but another rain garden being installedAgriculture, 2005). (2005) about 1 mi north of the existing rain garden does not have the permeable substrate and requires special engineering
  10. 10. 4 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04 94°00 93°45 93°30 93°15 93°00 92°45 r ISANTI ve Ri North & South SHERBURNE CHISAGO Center Lakes Rum 10 Chisago r ve Lake Ri 35 ANOKA Forest Lake 45°15 94 Big Croix Pelican 65 Marine Lake r Rive Lake WRIGHT M iss iss ip Blaine 35W pi Hugo St. Buffalo Lake ow Coon Rapids N. Cr F. Brooklyn Park Cr White R. Fridley ow Howard Bear Lake 35E Lake Maple Grove Brooklyn Center 694 HENNEPIN 12 94 WASHINGTON Plymouth RAMSEY r 45°00 C ix ve ro Minneapolis Ri ka Saint Paul Maplewood et on 94 Saint Louis ow n in River Park Cr M Minnetonka Woodbury . St Lake Minnetonka F. Lake Edina MC LEOD Waconia Richfield 494 S. Eden Prairie Bloomington 61 e Lak CARVER 35W Eagan 101 River 35E 21244°45 Burnsville Spring Lake 10 Apple Valley Chanhassen Lake r Pepin ta DAKOTA ve es o Ri n on in 169 SCOTT illi M rm Ve Lakeville 61 SIBLEY 52 er 35 Riv non44°30 Can Lake Byllasby LE SUEUR GOODHUE NICOLLET RICE Base from U.S. Geological Survey Digital data, 1:100,000, 1985 0 10 20 30 40 50 MILES U.S. Albers projection EXPLANATION 0 10 20 30 40 50 KILOMETERS PERCENT CLAY 0 to less than 8 8 to less than 16 MINNESOTA 16 to less than 24 Map area 24 to less than 32 RAIN-GARDEN STUDY SITEFigure 2. Location of rain-garden sampling sites and percentage of clay in soils in the Minneapolis-St. Paul metropolitan area of Min-nesota.
  11. 11. Introduction 5Table 1. Rain-garden sites sampled in the Minneapolis - St. Paul metropolitan area of Minnesota[Latitude and longitude: DD, degrees; MM, minutes; SS, seconds; NA, not applicable] Latitude Longitude Site identifier Site name Start date End date DDMMSS DDMMSS445149093365502 Rain garden lysimeter near Chanhassen, MN 445149 0933655 Aug. 2003 Nov. 2004445149093365503 Rain garden inflow near Chanhassen, MN 445149 0933655 Sep. 2003 Sep. 2004445150093365402 Rain garden background lysimeter near Chanhassen, MN 445150 0933654 Sep. 2003 Oct. 2004445150093365403 Rain garden outflow near Chanhassen, MN 445150 0933654 Aug. 2003 Sep. 2004450943092593901 Rain garden well at Hugo, MN 450943 0925939 Aug. 2002 Nov. 2004450943092593902 Rain garden lysimeter at Hugo, MN 450943 0925939 Aug. 2002 Nov. 2004450943092593903 Rain garden inflow at Hugo, MN 450943 0925939 Jul. 2002 Sep. 2004450946092593901 Rain garden background well at Hugo, MN 450946 0925939 Jun. 2002 Nov. 2004450946092593902 Rain garden background lysimeter at Hugo, MN 450946 0925939 Aug. 2002 Nov. 2004450946092593903 Rain garden outflow at Hugo, MN 450946 0925939 NA NA443914093171801 Rain garden well at Lakeville, MN 443914 0931718 Sep. 2002 Nov. 2004443914093171802 Rain garden lysimeter at Lakeville, MN 443914 0931718 Sep. 2002 Nov. 2004443914093171803 Rain garden inflow at Lakeville, MN 443914 0931718 Sep. 2003 Sep. 2004443920093173501 Rain garden background well at Lakeville, MN 443920 0931735 Oct. 2002 Nov. 2004443914093173602 Rain garden background lysimeter at Lakeville, MN 443914 0931736 Sep. 2002 Nov. 2004443920093173503 Rain garden outflow at Lakeville, MN 443920 0931735 NA NA445643093253801 Rain garden well near Minnetonka, MN 445643 0932538 Nov. 2003 Oct. 2004445643093253802 Rain garden lysimeter near Minnetonka, MN 445643 0932538 Aug. 2003 Oct. 2004445643093253803 Rain garden inflow near Minnetonka, MN 445643 0932538 Aug. 2003 Oct. 2004445645093254001 Rain garden background well near Minnetonka, MN 445645 0932540 Aug. 2003 Oct. 2004445645093254002 Rain garden background lysimeter near Minnetonka, MN 445643 0932538 Aug. 2003 Aug. 2004445645093254003 Rain garden outflow near Minnetonka, MN 445645 0932540 Aug. 2003 Oct. 2004445512092564401 Rain garden well near Woodbury, MN 445512 0925644 Oct. 2002 Oct. 2004445512092564402 Rain garden lysimeter near Woodbury, MN 445512 0925644 Aug. 2003 Oct. 2004445512092564403 Rain garden inflow near Woodbury, MN 445512 0925644 Jun. 2003 Sep. 2004445516092563801 Rain garden background well near Woodbury, MN 445516 0925638 Oct. 2002 Oct. 2004445516092563802 Rain garden background lysimeter near Woodbury, MN 445516 0925638 Aug. 2003 Aug. 2004445516092563803 Rain garden outflow near Woodbury, MN 445516 0925638 Jun. 2003 Sep. 2004
  12. 12. 6 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04to encourage infiltration (Keith Nelson, Lakeville City Engi- consisted of runoff from a series of parking lots interspersedneer, oral commun., 2005). Soils maps indicate that both the with vegetated strips. The inflow was sampled by using aestablished and under-construction rain gardens are situated in float-activated sampler at the primary point where storm-waterthe same type of soils. Other considerations such as the water- runoff enters the rain garden. Overflow samples from thistable altitude also may affect infiltration and may need to be site are unique because they come from a drain tile installedevaluated on a site-specific basis. beneath the rain garden. The drain tile empties into a culvert The closest resemblance to a soil test available for this that serves as an overflow during major rainfall events. Datastudy was the lithologic logs for the monitoring wells that collection from this site was complicated by several factors.were installed at the rain-garden sites (table 2). Materials Arboretum staff attempted to maintain flowering plants in theencountered ranged from coarse sand to clay, with some gravel rain garden that required frequent watering, which could pro-and cobbles. The least-permeable material was present at duce overflow without inflow. The source of this added waterChanhassen, where wells were not installed because the clay was assumed to be the public water supply for the arboretum.was impervious to water transport. In addition, ongoing construction surrounded the site during Many of the rain gardens have multiple inflows because the sampling period. The effects of construction on runoffwater can come from several surrounding impervious sur- loading to the rain garden were not measured during the study.faces. Rain gardens generally are situated in low-lying areas. The Hugo site (fig. 4) is underlain by sand and gravelBecause it is difficult to install multiple intakes that can col- that is part of the Anoka Sand Plain. Runoff to the rain gardenlect a representative, proportionate sample from each of the is from the parking lot and from the roof of Hugo City Hall.inflows, one inflow was selected at each site that was expected Other nearby areas also may contribute runoff to the rainto provide the largest, most representative inflow to that rain garden. Ground water is assumed to flow toward the site fromgarden. These largest inflows are assumed to be the appropri- the northwest, an area that consists mostly of gravel roads andate sampling sites for this qualitative study. athletic fields. The background well and lysimeter are located The typical site installation (fig. 1) encompassed two in this area. No overflow from the rain garden was observedautomatic samplers. One was configured to collect water at the during the study and consequently an overflow sample was notprimary site of inflow. The other was configured to sample collected.the overflow when sufficient water passed into and through The Lakeville site (fig. 5) is underlain by a mixture ofthe rain garden to generate overflow. A well and lysimeter sandy soils and glacial till. The site was in a state of transitionwere installed within the rain garden to measure the quality of during the study. Much of the contributing drainage area con-water that might infiltrate from the water ponded within the sists of a townhouse development that was under constructionrain garden. A well and lysimeter also were installed in an throughout the sampling period. The effects of constructionarea believed to represent background conditions that are not on the volume and quality of runoff to the rain garden wereinfluenced by infiltration from the rain garden. not measured during the study. The background observation The Chanhassen site (fig. 3) is located within the parking well and lysimeter were located away from areas of construc-lot of the University of Minnesota Landscape Arboretum. The tion, but generally near road rights of way that could influencesite is underlain by clay-rich soils derived from glacial till. the quality of water recharged to those monitoring points.Consequently, observation wells were not installed. Inflow Figure 3. Rain-garden configuration at Chanhassen, Minnesota.
  13. 13. Introduction 7Table 2. Lithologic log of wells installed at rain-garden sites in the Minneapolis - St. Paul metropolitan area of Minnesota Well Minnesota depth Depth range Hard- (feet) Site identifier Site name unique (feet be- Material drilled Color ness number low land surface) From To443914093171801 Rain garden well at 620719 15 Coarse Sand Brown 0 9 Lakeville, MN Silty Gray Clay Gray 9 15443920093173501 Background well at 685801 24 Top Soil Dark Brown 0 2 Lakeville, MN Clay Brown 2 3 Sand Gravel with Clay Brown 3 6 Sandy Gravel Brown 6 10 Silty Sand Gravel with Brown 10 14 Cobbles Silty Sand without Brown 14 17 Cobbles Silty Sand, Gravel, Brown 17 21 Cobbles Gray Clay Brown 21 24445512092564401 Rain garden well near 685807 11 Fine Silty Sand Brown Medium 0 11 Woodbury, MN445516092563801 Background well near 685803 20 Top Soil Black 0 2 Woodbury, MN Clay Brown 2 7 Sandy Clay Red 7 13 Sandy Clay Red 13 17 Sand Red 17 19 Clay Brown 19 20445643093253801 Rain garden well near 620660 6 Organic Peat Black Soft 0 1 Minnetonka, MN Medium Coarse Sand Brown Soft 1 6445645093254001 Background well near 620659 12 Medium Sand Brown Soft 0 12 Minnetonka, MN450943092593901 Rain garden well at 620717 19 Organic Topsoil 0-0.5 Black 0 1 Hugo, MN feet Fine Sand Brown Soft 1 19450946092593901 Background well at 620718 22 Topsoil 0-0.5 feet Black 0 1 Hugo, MN Medium Sand Brown Soft 1 22445159093365503 1 Rain garden well cancelled 22 Fill Sand Brown Soft 0 2 near Chanhassen, Clay Brown Hard 2 22 MN1 Well was never installed.
  14. 14. 8 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04 Hugo City Hall site Road Background well and lysimeter Rain-garden infiltration N basins e ag ain Dr City Hall Parking lot Inflow Inflow 61Overflowstormsewer Rain-garden Under construction well and lysimeter 170 feet Lysimeter Well Well and lysimeter Inflow Overflow Automatic sampler OverflowFigure 4. Rain-garden configuration at Hugo, Minnesota.
  15. 15. Introduction 9Figure 5. Rain-garden configuration at Lakeville, Minnesota.
  16. 16. 10 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04 Figure 6. Rain-garden configuration at Minnetonka, Minnesota.. The Minnetonka site (fig. 6) is underlain by soils with a stituents by the rain garden as well as by the unsaturated zone.relatively high percentage of clay (fig. 2). Much of the con- The suction lysimeters and water-table wells generally weretributing drainage area near the rain garden consists of athletic sampled monthly for indicator constituents during the sam-fields and a maintenance facility. However, the majority of pling period, although it was not uncommon for some of theserunoff to the site is from general-use parking. This parking sampling points to have insufficient water for collection andinflow was the only one of five inflows to this rain garden that analysis.could be routinely sampled because of technical consider- Automated samplers were designed to obtain samplesations described previously. of inflow to and overflow from the rain gardens. Samplers The Woodbury site (fig. 7) is underlain by soils with a were programmed to collect initial runoff and to sample at arelatively large percentage of clay (fig. 2). The rain garden reduced frequency as the runoff continued during a rainfallreceives runoff from nearby roadways, housing developments, event. One minute after sensing runoff, 1.6 liters of waterand a series of small impoundments. The site was unusual were collected. An additional 1.6 liters of water were col-because overflow commonly occurred during periods of little lected after 2 minutes had elapsed since the previous sample,or no runoff, indicating that it was supplied from upstream continuing until three samples had been collected. A fourthponds or ground-water inflow. sample of 0.4 liter was collected after 10 minutes had elapsed since the sampler first was activated by an event. The remain- ing samples of 0.4 liter each were collected every 5 minutesMethods until all the bottles were filled. If runoff stopped before all the bottles in the sampler were filled, a partial sample was An idealized concept of sampling points at each site is collected. If the runoff continued beyond the capacity of theshown in figure 1. The installation included inflow and over- sampler, that water was not sampled.flow samplers, two ground-water observation wells completed Individual site-monitoring installations varied because ofjust below the water table, and two soil-moisture lysimeters conditions specific to each site. Wells were not installed at theinstalled in the unsaturated zone. Chanhassen site because the subsurface consisted primarily of Suction lysimeters were installed in the unsaturated zone clay, and wells would not have yielded water. Other compli-underlying each rain garden to facilitate collection of infiltra- cations included delays in site-monitoring installation due totion water before it reached the water table. Water-table obser- delays in rain-garden construction. Also, some sites were sovation wells were installed near the middle of each rain garden effective in attenuating runoff, such as the Hugo site, that littleto sample water reaching the underlying aquifer. To provide or no overflow occurred and few or no samples of overflowbackground (presumably upgradient) chemical information, were collected.an additional suction lysimeter and well were installed some Wells and lysimeters were installed in a manner con-distance from the rain garden. Comparison of the chemical sistent with the guidance provided by Wood (1976). Inflowdata from the surface runoff (inflow) samplers and the suction and overflow automatic samplers were installed according tolysimeters with the data from the water-table wells provides manufacturer’s recommendations (Isco, Inc., 1996, instruc-information regarding the attenuation of the chemical con-
  17. 17. Water Quality at Rain-Garden Sites 11 Figure 7. Rain-garden configuration at Woodbury, Minnesota.tion manual for 3700 portable sampler, 209 p.). Samples were The areal extent of the area of study also resulted incollected and processed by using standard methods developed substantial variability in rainfall. Local rainfall sometimesand published by the USGS. A complete list of the techniques produced deluge conditions at a site while leaving other sitesthat were adapted to collect and process samples for this without precipitation. The density of real-time rainfall moni-study is available as part of the USGS Techniques of Water- toring was not sufficient to provide adequate information forResources Investigations publication series (U.S. Geological ideal timing of site visits in several instances.Survey, variously dated) that can be accessed at http://water. Approximately 15 percent of all water-quality samplesusgs.gov/pubs/twri. were collected for quality-assurance (duplicates, blanks, Samples collected by the automatic samplers were trans- splits) purposes. All water-quality samples were collectedported to the USGS Water Science Center of Minnesota and and analyzed by using the USGS quality-assurance protocolscomposited into a churn splitter for collection of representative documented at http://water.usgs.gov/owq/quality.html andsubsamples for analysis. Field values were determined from http://wwwrcolka.cr.usgs.gov/uo/proposals/ Tables1&2DQOs.these subsamples. Water samples were filtered and preserved, pdf. Coding of water-quality samples followed the proceduresand analyzed at the USGS National Water Quality Laboratory documented at http://ar.water.usgs.gov/nawqa/sample-coding/using the methods described in Fishman and Friedman (1989). outline.html.Samples were analyzed for constituents listed in tables 3through 7. The data collected for this study are available from twosources, including the annual USGS water-resources data WATER QUALITy AT RAIn-GARDEnreports (Mitton and others, 2003, 2004, and 2005), which SITESalso are published electronically on the USGS Water ScienceCenter of Minnesota website at http://mn.water.usgs.gov. Data Periods without water-quality data resulted from the lackalso can be retrieved from the USGS National Water Informa- of runoff and recharge that occurs during the winter. Personstion System website (NWIS-Web) at http://waterdata.usgs. were dispatched on several occasions to manually samplegov/nwis. snowmelt runoff because the automatic samplers likely would The timing of the sampling varied. Some samples were have been damaged by freezing conditions, but water samplescollected early in the study, as the sites were being established rarely were collected. When hydrographers arrived, runoffand instrumented (table 1). A period of less-than-normal generally was not sufficient to provide adequate sample vol-precipitation ensued after the installation that resulted in few ume. Because most of these sites drained roadways or parkingor no samples. Although sites were routinely visited, site and lots, any snow or other frozen precipitation that had accumu-weather conditions sometimes prevented collection of water lated typically was pushed or transported to areas where it didfor chemical analysis. In some instances lysimeters were dry not contribute to the inflow of the rain garden.and ground-water levels had dropped below the screened inter- After the sites had been established and the weatherval of the wells because of the extended dry period. became more conducive to generating runoff, more samples were collected. Substantial variability among the sites resulted
  18. 18. 12 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04from differences in site conditions and rain-garden design. samples indicate that fertilizers may have been applied to theThe data allow for general observations about each of the sites rain garden during the course of this study.and about differences among the individual sites. The soils beneath this site had a high clay content, which Median concentrations for the data collected from all five precluded installation of monitoring wells. Therefore, noof the rain-garden sites are shown in tables 3 through 7. These data are available to assess the effects of the Chanhassen raintables also show the approximate number of each type of con- gardens on ground-water quality.stituent measured from each of the media sampled. Individual Data from the Chanhassen site indicate that the chemis-sample numbers used to compute the median varied depending try of each sampling site (inflow compared to overflow, andon a variety of factors, such as availability of adequate water to background lysimeter compared to rain-garden lysimeter) con-complete the intended analysis. Water-quality results are sum- verges over time. Throughout much of the study, nitrogen andmarized in this section. total phosphorus concentrations were lower in the overflow as compared to the inflow, indicating that the rain garden was assimilating much of the nutrients that might have otherwiseChanhassen been transported to the overflow. Water quality had changed little from inflow to overflow during the most recent sampling The median specific-conductance value of the over- visits as determined from measurements of specific conduc-flow at the Chanhassen site was much higher than that of the tance, pH, and concentrations of chloride, dissolved solids,inflow (table 3). The increase may be attributed to additional, and dissolved phosphorus. This indicates that the Chanhassenunsampled storm-water runoff from the parking lot and (or) rain garden may approach a state of equilibrium with respectinfiltration through the substrate, which leached minerals to the to quality of inflow and overflow for some constituents.drain tile and was sampled as overflow. Total suspended solidswere retained by the rain garden to levels less than the 10 mg/Lmethod reporting limit for this measurement. The concentra- Hugotion of most nitrogen species measured at the rain-gardenoverflow decreased by an order of magnitude from that mea- Samples from several inflow events were collected, butsured at the inflow. The median chloride and dissolved-solids overflow samples never were observed or sampled. Thisconcentration increased from inflow to overflow. Median dis- indicates that storage within the rain gardens was adequate tosolved phosphorus concentrations generally were similar from assimilate the inputs, and that infiltration to the subsurface wasinflow to overflow, but median total phosphorus concentrations effective.decreased from inflow to overflow. Samples from the background and rain-garden lysimeters The background lysimeter was frequently dry, so few had similar median values of constituents measured (table 4).samples were collected from that site. When both background Median chloride concentrations in the rain-garden lysimeterand rain-garden lysimeters were sampled concurrently, pH and were about half those measured in the background, indicatingconductance were similar, and nutrient concentrations were some dilution effect.similar but near method reporting limits. Concentrations of The background and rain-garden wells had similar pHnitrogen and phosphorus greater than median values in some and nutrient concentrations. Chloride concentrations in sam- ples collected from the background and rain-garden wells were Table 3. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Chanhassen, Minnesota, 2002-04 [cm, centimeter; mg/L, milligrams per liter; °C, degrees Celsius; N, nitrogen; P, phosphorus; n/a, no samples for that constituent] Specific nitrogen, Solids, Residue nitrogen, nitrogen, Phos- Sample location pH, conduct- am- nitrogen, Phos- residue total at nitrite + nitrite, phorus, (approximate number water, ance Chloride monia + ammonia, phorus, at 180 105 oC, nitrate, dis- dis- of samples; may be whole, (micro- (mg/L as organic, dissolved total o C, dis- sus- dissolved solved solved fewer for some field siemens/ Cl) total (mg/L as (mg/L solved pended (mg/L as (mg/L as (mg/L measurements) (units) cm at (mg/L as n) as P) (mg/L) (mg/L) n) n) as P) 25°C) n) Inflow composite (5) 7.8 176 3.2 198 190 3.6 0.85 1.15 0.05 0.04 0.29 Overflow composite (6) 7.5 656 17 426 10 .43 .04 .15 .01 .04 .04 Background lysimeter (2) 7.8 725 n/a n/a n/a .31 .04 .06 .01 .03 .03 Background well (0) n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Rain-garden lysimeter (2) 7.5 645 10 n/a n/a .32 .04 .10 .01 .06 .06 Rain-garden well (0) n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a
  19. 19. Water Quality at Rain-Garden Sites 13 Table 4. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Hugo, Minnesota, 2002-04 [cm, centimeter; mg/L, milligrams per liter; oC, degrees Celsius; N, nitrogen; P, phosphorus; n/a, no samples for that constituent] Specific nitrogen, Solids, Residue nitrogen, nitrogen, Phos- Sample location pH, conduct- am- nitrogen, Phos- residue total at nitrite + nitrite, phorus, (approximate number water, ance Chloride monia + ammonia, phorus, at 180 105 oC, nitrate, dis- dis- of samples; may be whole, (micro- (mg/L as organic, dissolved total o C, dis- sus- dissolved solved solved fewer for some field siemens/ Cl) total (mg/L as (mg/L solved pended (mg/L as (mg/L as (mg/L measurements) (units) cm at (mg/L as n) as P) (mg/L) (mg/L) n) n) as P) 25°C) n) Inflow composite (8) 8.0 118 3 65 45.5 0.95 0.18 0.45 0.03 0.19 0.3 Overflow composite (0) n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a Background lysimeter (4) 9.3 174 3.3 n/a n/a .35 .04 2.0 .01 .04 .04 Background well (10) 6.7 213 43 138 n/a .11 .04 2.0 .01 .07 .08 Rain-garden lysimeter (7) 9.2 164 1.6 n/a n/a .35 .04 1.28 .01 .09 .07 Rain-garden well (10) 6.9 67.5 .95 56 n/a .15 .04 .79 .01 .06 .06substantially different and diverged over time, indicating that evidence of pesticide application near the rain garden (L.runoff entering the rain garden may dilute existing concentra- Gryczkowski, U.S. Geological Survey, oral commun., 2005),tions of dissolved salts as recharge is focused on the ground which might indicate that fertilizers also were applied near thewater immediately beneath the rain gardens (G.N. Delin, U.S. site and eventually seeped into the subsurface.Geological Survey, oral commun., 2005). The specific-con- Water from the background well at Lakeville generallyductance value in samples from the background well increased had much higher specific-conductance values and concen-during this study and decreased in samples from the rain-gar- trations of chloride and measured forms of nitrogen thanden well. Chloride concentrations showed similar trends to were measured in the rain-garden well (table 5). Dissolvedspecific conductance during this study. and total phosphorus concentrations were comparable in the background and rain-garden wells. The maximum value of phosphorus measured in the rain-garden lysimeter coincidedLakeville with the maximum measured in the rain garden well. Other than the peak phosphorus concentrations observed Samples from several inflow events were collected, but in the rain-garden well and lysimeter during September 2004,overflow samples never were observed or sampled. This no trends were apparent. However, increases during theindicates that storage within the rain gardens was adequate to study period (September 2002 through November 2004) wereassimilate the inputs and that infiltration to the subsurface was apparent in the background well and lysimeter. Specific-con-effective. On one occasion a sample of standing water from ductance values and concentrations of chloride and nitrite plusthe rain garden was collected and is referred to as an overflow nitrate nitrogen generally increased during this study. Thesample. reasons for these increases are not known but could be related Specific conductance and concentrations of chloride and to ongoing periods of reduced precipitation with less dilution.nutrients (nitrogen and phosphorus species) in inflow gener- They also could be the result of roadway runoff because bothally were low (table 5). A runoff event sampled on July 30, background sites are located near heavily used transportation2004, had a suspended-solids concentration of 230 mg/L and routes.concentrations of several nutrients, including ammonia plusorganic nitrogen and total phosphorus, that also were thehighest measured during this study. Because more conserva- Minnetonkative components of runoff such as specific conductance andchloride concentration generally did not vary, it is assumed The rain garden in Minnetonka typically had samples ofthat nutrient-enriched soils exposed during ongoing construc- both inflow and overflow. The large volume of runoff (inflow)tion washed into the rain garden during this event. relative to the size of this rain garden resulted in a relatively The rain-garden lysimeter had specific-conductance short retention time. The field and laboratory water-qualityvalues and chloride concentrations that were much lower than measurements of inflow as compared to overflow providedthose measured in the background lysimeter, with nitrogen results that were very similar during concurrent samplings.concentrations generally following the same pattern. How- The rain garden frequently retained water, indicating thatever, dissolved and total phosphorus concentrations were infiltration to the ground-water system was limited. Determi-slightly higher in the rain-garden lysimeter than in the back- nation of the soil characteristics beneath this rain garden mightground lysimeter. During a site visit a hydrographer observed indicate whether drainage is adequate. The layer of organic
  20. 20. 14 Effects of Rain Gardens on the Quality of Water in the Minneapolis-St. Paul Metropolitan Area of Minnesota, 2002-04 Table 5. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Lakeville, Minnesota, 2002-04 [cm, centimeter; mg/L, milligrams per liter; oC, degrees Celsius; N, nitrogen; P, phosphorus; n/a, no samples for that constituent] Specific nitrogen, Solids, Residue nitrogen, nitrogen, Phos- Sample location pH, conduct- am- nitrogen, Phos- Chlo- residue total at nitrite + nitrite, phorus, (approximate number water, ance monia + ammonia, phorus, ride at 180 105 oC, nitrate, dis- dis- of samples; may be whole, (micro- organic, dissolved total (mg/L o C, dis- sus- dissolved solved solved fewer for some field siemens/ total (mg/L as (mg/L as Cl) solved pended (mg/L as (mg/L as (mg/L measurements) (units) cm at (mg/L as n) as P) (mg/L) (mg/L) n) n) as P) 25°C) n) Inflow composite (6) 7.4 99 1.6 95.5 14 1.3 0.23 0.32 0.02 0.15 0.20 Overflow composite (1) 7.6 63 .5 54 10 .52 .04 .06 .01 .19 .2 Background lysimeter (8) 8.1 2,980 24 n/a n/a 1.1 .04 9.40 .01 .1 .09 Background well (10) 7.0 1,850 310 661 n/a .26 .04 3.9 .01 .04 .04 Rain-garden lysimeter 8.4 298 2 n/a n/a .33 .04 1.6 .01 .14 .13 (10) Rain-garden well (9) 7.6 307 5.9 147 n/a .25 .04 .64 .01 .04 .04peat shown in the well log (table 2) at the point where the only enough to provide partial results. That sample had aobservation well was installed would reduce infiltration if it specific-conductance value of 2,110 µS/cm. The rain-gardencovered the bottom of the rain garden. lysimeter generally yielded sufficient water for most analy- Median values of specific conductance and concentra- ses and had lower specific-conductance values and nutrienttions of chloride and dissolved solids at the Minnetonka site concentrations as compared to those values measured in thewere greater in the overflow as compared to the inflow (table background lysimeter.6). This may be the result of evapotranspiration in the rain The background well at this site had nutrient concentra-garden or inflow from contributing areas that were not sam- tions that generally were low and often near the detectionpled for this study, but were sampled as part of the overflow limit. However, much of the nitrogen measured was in thethat combined water from several inflow sources. form of nitrate nitrogen (the nitrite concentration was negli- The concentration of suspended solids and most nitro- gible), with concentrations that ranged from 2.8 to 6.6 mg/Lgen compounds was less in overflow as compared to inflow, with a median of 5.72 mg/L. Although this is predominantlyindicating sedimentation, dilution, or uptake or attenuation by a residential area, these concentrations could be indicative ofvegetation. The median concentration of total and dissolved fertilizer inputs in agricultural regions that have permeablephosphorus was similar in inflow and overflow. soils (Hanson, 1998). The background lysimeter at this site yielded only one One complete analysis and one partial analysis weresample sufficient to provide water for analysis, and that was done (because of insufficient water collected from the well) Table 6. Median values of selected physical properties, chemical constituents, and nutrient species of water from the rain-garden site in Minnetonka, Minnesota, 2002-04 [cm, centimeter; mg/L, milligrams per liter; oC, degrees Celsius; N, nitrogen; P, phosphorus; n/a, no samples for that constituent] Specific nitrogen, Solids, Residue nitrogen, nitrogen, Phos- Sample location pH, conduct- am- nitrogen, Phos- Chlo- residue total at nitrite + nitrite, phorus, (approximate number water, ance monia + ammonia, phorus, ride at 180 105 oC, nitrate, dis- dis- of samples; may be whole, (micro- organic, dissolved total (mg/L o C, dis- sus- dissolved solved solved fewer for some field siemens/ total (mg/L as (mg/L as Cl) solved pended (mg/L as (mg/L as (mg/L measurements) (units) cm at (mg/L as n) as P) (mg/L) (mg/L) n) n) as P) 25°C) n) Inflow composite (9) 7.9 134 9.6 134 172 2.4 0.45 0.59 0.03 0.13 0.34 Overflow composite (6) 8.1 270 17 183 66 1.9 .13 .46 .04 .14 .35 Background lysimeter (1) 8.0 2,110 n/a n/a n/a 4.2 n/a n/a n/a n/a .14 Background well (8) 7.5 574 8.4 n/a n/a .35 .04 5.72 .01 .04 .04 Rain-garden lysimeter (7) 8.2 964 44 n/a n/a .92 .04 1.13 .01 .07 .08 Rain-garden well (1) 12.1 1,940 21 n/a n/a n/a .53 1.15 .54 .04 .09

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