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PowerPoint for University of Guam course, EV 511-01 Spring 2014. Professor John Jenson Northern Guam Lens Aquifer
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- 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….