NGLA presentation EV 511 Spr 2014: Nothern Guam Lens Aquifer Professor John Jenson
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This document discusses the Northern Guam Lens Aquifer and sustainable groundwater management. It provides background on the aquifer system, including its geology, rainfall patterns, and freshwater lens dynamics. It summarizes previous studies that estimated the aquifer's sustainable yield and compares those estimates to current extraction rates. The document cautions that sustainable yield is not a fixed value and that proper management requires observing trends over time. It argues that the focus should be on economically meeting water needs, not just supply, through approaches like pricing, conservation, and considering alternative technologies and their costs.
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NGLA presentation EV 511 Spr 2014: Nothern Guam Lens Aquifer Professor John Jenson
- 1. The Northern Guam Lens Aquifer Carbonate Island Karst Model (CIKM) Sustainable Yield EV511 Groundwater Module Lecture #8
- 4. 0 5 10 15 20 25 30 35 40 Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec MeanMonthlyRainfall(cm). Mean Monthly Rainfall (1982-1995) Dry Season Wet Season
- 5. 0.17 0.28 0.33 0.22 0.01 0.11 0.27 0.36 0.25 0.00 0.26 0.30 0.27 0.16 0.01 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 less than 0.6 0.6 to 2 2 to 5 5 to 20 more than 20 Daily total rainfall (cm) Proportionoftotalrecordedrainfall. Annual Wet season Dryseason Distribution of Rainfall Intensity
- 6. discharging fresh water Northern Guam Lens Aquifer percolating water infiltrating rain water sea water fresh water lens volcanic basement rock limestone bedrock Annual Rainfall 2.4 m (~100 in) Annual Water loss 0.8 m (~33 in) Annual Recharge 1.6 m (~67 in) = ~310 mgd Extraction ~43 mgd = 14% est. recharge
- 7. discharging fresh water Northern Guam Lens Aquifer percolating water infiltrating rain water sea water fresh water lens volcanic basement rock limestone bedrock Annual Rainfall 2.4 m (~100 in) Annual Water loss 0.8 m (~33 in) Annual Recharge 1.6 m (~67 in) = ~310 mgd Extraction ~43 mgd = 14% est. recharge
- 8. #8 Optimizing Well Locations Best practices for production volume and water quality Topographic Map of Basement Volcanic Rock Vann, 2000 Habana, 2010
- 9. Agafa Gumas Subbasin Mangilao Subbasin Finegayan Subbasin Yigo Subbasin Agana Subbasin Andersen Subbasin N 2 0 2 4 Miles Chloride Profile Wells Basement Topography
- 10. Dye trace study of Harmon Sink to Tumon & Agana Bays Joint project with Guam EPA: 2000-2002 1 0 1 2 Kilometers N EW S 55000 55000 50000 50000 55000 55000 '] '] #S #S Tamuning-Yigo FaultTamuning-Yigo Fault Tumon Bay Agana Bay Tamuning Jonestown Tumon Harmon Industrial Park Guam International Airport Oka Point Marine Drive 3.63.6 3.73.7 3.83.8 3.93.9 4.04.0 Fujita Hotel# Harmon Sink Injection Pit Airport Injection Well Airport Sampling Well Alupang Beac h Stream 130 m/d Dungca's & BBQ Beach Springs 350 m /d D ung ca's Stre am & S prin g 6 50 m /d Ypao & Pacific Island Club Springs 70-94 m /d Pacific Islands Club Spring 175 m /d Airport Sampling Well 38 m/d Harmon Sink Sampling Well 38 m /d
- 11. • Defining features 1. Young limestone & eogenetic karst 2. Freshwater-saltwater mixing at base and margin of lens 3. Glacioeustasy has moved the lens up and down - Tectonic uplift and subsidence may have also occurred - Still-stands affect aquifer development 4. Basement-sea level-surface relationships affect aquifer development 1. Simple 2. Carbonate covered 3. Composite 4. Complex Carbonate Island Karst
- 13. Closed Depressions • Sinkholes and dolines • Structural control • Lithologic control • Quarries and ponding basins
- 14. Recharge Features • Argillaceous vs. pure limestone aquifers • Distributed vs. concentrated recharge • Sinkholes, ponding basins, injection wells
- 15. Agana Sub-Basin • Argillaceous Mariana LS • Alifan LS • “Miniature” classic karst • Classic karst plumbing….
- 16. limestone aquifer water table sea level volcanic basement Chloride Benchmarks Safe Drinking Water guideline 250 mg/l Saltwater 19,000 parabasal range < 30 mg/l < 30 parabasal water saltwater intrusion > 150 mg/l > 150mixing zone saltwater toe range > 30 to 70 mg/l < 70 saltwater toe basal range > 70 to < 150 mg/l basal water < 150
- 17. Surface Contaminants Potentially Carried by Recharging Water industrial spills agricultural runoff storm water coastal contamination septic tanks & sewage spills
- 18. Aquifer Studies: Previous SY Estimates Aquifer Sub-Basins and Estimated Sustainable Yields Reserve Production (1982 Study)80 mgd (1991 Study) 1982 SY Estimates 1998 Production • 1982 Northern Guam Lens Study: 57 MGD – First comprehensive field, lab, and geophysical study – $1.2M, 3-year effort – GEPA, WERI, CDM (Mink) – Still the departure point…. • 1991 Update (Mink): 80 MGD • Current fresh groundwater production*: 43 mgd – 54% of 1991 SY est. - GWA: 37 (2010) - AF: ~3 (2008) - Navy: ~2 (2008) - All others (private) ~1 (2008) ~20% recharge ~25% recharge *GEPA database ~14% recharge
- 19. Having said all that… The Myth of “Sustainable Yield”
- 20. The Myth of “Sustainable Yield” • “Sustainable yield” as a percent of recharge is an old but persistent misconception – Still provides a useful “rule of thumb”…but it’s not the real determinant • Thiess first pointed out the error in 1940 – Even today, even many hydrologists still don’t get it – Recent papers by prominent hydrologists notwithstanding…. • One use of models is to help evaluate how pumping changes the system – In terms of both quantity and quality • But models cannot replace the need for observed data – In fact, they require data—lots of it! • Proper management – Comes from observing the trends and responding according – Requires effective inter-agency cooperation and decision-making - Which requires an inter-agency framework for ongoing consultation • Finally, it addresses only the supply question; overlooks the demand aspect of water management
- 21. Economics of Sustainability (101) • Higher salinity is the natural consequence of production • Green* technologies seem at first to be the obvious choice – But they may be (and usually are) more expensive… - Unless the incremental cost is lower in the long run… • The affordable limit may change – Upward if the economy is growing… • Regulatory limits should be routinely re-examined – And adjusted based on observable or predictable consequences… • “Sustainable” really means “economical” – i.e., meeting the objective with least expenditure of resources Production Salinity regulatory limitCost affordable limit *Green = satisfies a particular environmental objective that is widely acknowledged as desirable vertical wells tunnel wells High start-up, high ops cost desalination Low and high switch places
- 22. Sustainable Use • The old question…. – “What is the sustainable yield…?” • Is the wrong question! • The right question…? – “How can we economically meet the potable water needs of the entire community?” - Demand—just as important as supply – Pricing structure – Conservation incentives - Supply—how much of a given quality at a given cost? – Alternative technologies…and their costs – Modeling can help—if the model is accurate – Is only as good as the data – The right kinds of data, and enough of them – Sufficiently accurate and precise to do the job….