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HomeMy WebLinkAbout7d Long Term Groundwater Sustainalbility for Shakopee7d SHAKOPEE PUBLIC UTILITIES MEMORANDUM TO: SHAKOPEE PUBLIC UTILITIES COMMISSSIO FROM: JOHN R. CROOKS, UTILITIES MANAG SUBJECT: LONG TERM GROUND WATER SUSTAI ILITY DATE: MAY 1, 2014 Shakopee's water supply has been an issue of concern for many years. With the dramatic increases in growth to Shakopee that have taken place since the opening of the Bloomington Ferry Bridge, SPUC has been able to effectively serve the electric and water needs of our community and surrounding areas. In 2004 the Utilities wanted to create a region specific groundwater model that would simulate continued growth while also addressing potential future annexation by the City and the potential sale of up to 3 million gallons a day MGD to the City of Savage. Savage was experiencing severe restrictions imposed by the DNR due to unique calciferous Savage Fen an endangered plants that exist in the Fen. The model was run under many scenarios, some best case and some worst case. Attached is the executive summary of the report generated from the groundwater model. Recent presentations by the Met Council have shown the sustainability, of the Jordan Aquifer to be in question and much different than the findings in the region specific model. Also in their presentation to City Council members during a recent meeting at the SCALE Facility, several inaccuracies were also discussed, including arsenic and nitrate concerns. Aquifer Sustainability Study Prairie du Chien - Jordan Aquifer Shakopee, Minnesota w SEH No. A- SHPUC0501.00 July 2005 v SEH Muhidlsciplined. Single Source. Trusted solutions for more than 75 years. Executive Summary Short Elliott Hendrickson, Inc. (SEH ®) was retained by the Shakopee Public Utilities Commission (SPUC) to complete this aquifer sustainability study for the Prairie du Chien - Jordan bedrock aquifer. This aquifer is the primary source water aquifer utilized by the SPUC for its public water supply system serving the City of Shakopee. In the Shakopee area, other options for water supply are limited — the deeper Franconia-Ironton- - Galesville bedrock aquifer is not highly productive and the Mount Simon - Hinckley Aquifer can not be used under current state law. Shallow sand and gravel aquifers are not prolific, and surface waters and their required treatment are cost prohibitive. Within the study area, the Prairie du Chien - Jordan Aquifer is also used for public water supply by the City of Prior Lake and the Mdewakanton Sioux Community, in addition to non - municipal uses (e.g. commercial and industrial businesses, golf courses, and individual domestic - supply wells). The demand for public water supply in Shakopee is likely to increase over the next 10 years to meet the ,. growth and development of the City and surrounding communities. This growth is projected not only for the city proper, but within neighboring communities such as Jackson and Louisville Townships that will likely depend on the SPUC to supply water for public, commercial, and industrial use. In addition, due to significant limitations in its ability to utilize local and regional aquifers for public water supplies, the City of Savage is proposing to purchase up to 3.0 million gallons per day (MGD) from the SPUC. The populations of the — Mdwakanton Sioux Community and the City of Prior Lake will also likely increase over time and require additional public water supplies. The purpose of this study was to evaluate whether the Prairie du Chien - Jordan Aquifer in the Shakopee area will be able to sustain these projected increases in its development and use. The purpose of the study was also — to assess how potential increases in future pumping of this aquifer may impact its capabilities, productivity, and long -term capacity to provide a source of groundwater. Sustainable water use is defined by the Minnesota Department of Natural Resources as the use of water for the needs of society, now and in the future, without unacceptable social, economic, or environmental consequences. This study involved a detailed review of the hydrogeological conditions in the study area and a compilation and analysis of future water use and demand projections. In addition, an assessment of potential decreases in — the aquifer's recharge due to future land use and development was performed. Two approaches were used to evaluate the viability and sustainability of the Prairie du Chien- Jordan Aquifer in the study area. In both approaches, conservative assumptions were employed to replicate worst -case scenarios. The first approach involved selecting an existing and recent computer -based groundwater flow model S; mulating the Prairie du Chien - Jordan Aquifer in the study area, refining it to a limited degree, and utilizing it to perform various future pumping and hydrogeologic scenarios. This method provided a measurement of the predicted decrease in groundwater elevation of the aquifer given the higher pumping rates and additional municipal wells necessary to meet the projected public water supply demands. Under this approach, the key observations from the modeling would include whether the elevations of the groundwater in the aquifer decrease below the top of the aquifer, or whether any portion of the aquifer in the model goes dry. Furthermore, the decrease in aquifer discharge to simulated surface water bodies, or increased recharge from surface water bodies could be estimated. The second method employed a numerical approach where the amount of groundwater storage in the Prairie du Chien -Jordan Aquifer is calculated, in addition to the estimated annual recharge to the aquifer. Then the projected withdrawal from the aquifer is compared to these values to relatively determine whether the aquifer may be overburdened. Aquifer Sustainabllity Study A- SHPUC0501.00 .. Shakopee Public Utilities The model indicates decreases in groundwater head ranging from less than one meter, to nearly 14 meters (46 b feet) under future pumping conditions. However, under the given assumptions and scenarios, it does not appear that the groundwater elevations of Layer 3 (representing the Jordan Sandstone) decrease to below the top of the aquifer, nor do any of the cells of the model in Layers 2 or 3 (combined representation of the Prairie du Chien -Jordan Aquifer) go dry. Figures are included depicting the results of modeling the various pumping scenarios. In addition, based on the model results, it appears that the increase in recharge from, or decrease in discharge to local and regional surface water bodies in the study area may change between 5 and 20 %. This suggests that to remain in a steady state condition, there will need to be an increase in recharge or a decrease in discharge for the aquifer to meet the increased pumping from the wells. Utilizing the numerical approach, assuming the annual aquifer recharge is only 7.6 billion gallons, the forecasted ultimate demand 10.52 billion gallons per year with supply to Savage or the 9.43 billion gallons per year without supplying Savage will exceed the estimated recharge. This suggests that to meet these demands, groundwater from aquifer storage would need to be utilized and could be depleted within 30 to 50 " years. If, however, the recharge to the aquifer is greater than 9.43 or 10.53 billion gallons per year, the aquifer storage may not be diminished over time. Nonetheless, as discussed at length in this report, this result is not to suggest that adverse outcomes may not occur due to this groundwater withdrawal, such as diminished — groundwater flows to surface water bodies and decreases in the groundwater head elevations of the aquifer. The following findings and opinions have been derived from this study, and are offered to the SPUC: ■ The Prairie du Chien -Jordan Aquifer in the study area is approximately 30,709 acres (1.34 billion square feet) and is typically 200 -220 feet thick. ■ In the study area, the groundwater in storage within the Prairie du Chien -Jordan aquifer is estimated to range from 88 to 188 billion gallons. ■ Recharge to the aquifer is estimated to range from 7.6 to 12.2 billion gallons per year. ■ Upon ultimate development and land use of the study area, the recharge to the aquifer may decrease 7 %. ■ Based on information from the SPUC 2004 Comprehensive Water Plan Update and data from the Metropolitan Council the following ultimate groundwater demand projections are forecasted for the Prairie du Chien - Jordan Aquifer in the study area: — City of Shakopee (including all potential annexation): 18.342 MGD — City of Prior Lake: 2.9 MGD — Mdewakanton Sioux Community: 0.5 MGD — Non - municipal use: 4.09 MGD ■ Presently the amount of groundwater removed from the Prairie du Chien - Jordan Aquifer in the study area for non - municipal use is estimated to be 4.09 MGD. In the ultimate water use A projections it was assumed that this amount of non - municipal pumping from the aquifer in the study area would remain constant into the future. ■ Assuming SPUC supplies 3.0 million gallons of water per day to the City of Savage, the total average day demand for groundwater from the Prairie du Chien- Jordan Aquifer in the study area is projected to be 28.842 million gallons. This is equivalent to approximately 10.5 billion gallons per year. Aquifer Sustainability Study A- SHPUC0501.00 Shakopee Public Utilities ■ Without supplying 3.0 MGD to the City of Savage, the ultimate total demand for groundwater from the Prairie du Chien - Jordan Aquifer in the study area is forecasted to be 9.4 billion gallons per year. ■ Groundwater flow modeling was performed to simulate various future aquifer pumping scenarios. In the scenarios it was assumed that SPUC would construct and utilize 12 additional public water supply wells, and the City of Prior Lake would construct and utilize five additional wells in the Prairie du Chien - Jordan Aquifer within the study area. ■ The groundwater flow modeling appears to suggest that the Prairie du Chien - Jordan Aquifer will remain in a fully- saturated condition (groundwater heads at or above the Jordan Sandstone [Layer 3]), even under reduced recharge and aquifer capacity conditions. However, the increased recharge from, or decreased discharge to, local and regional surface water bodies may be 5 -20 %. ■ Due to unavoidable and inherent complexities, and the number of assumptions required to address the issues, this study can not definitely provide a justification for or against the SPUC supplying 3.0 MGD of water to the City of Savage. With or without the additional 3.0 MGD supplied to Savage, the Prairie du Chien - Jordan Aquifer will be heavily taxed under the groundwater withdrawal assumptions for future, ultimate development in the study area. However, it is our professional opinion that this proposal could be accepted, at least in the short-term (i.e. 3 to 5 years), without permanent or long -term negative consequences or impacts to the aquifer in question. Based on the results of this study, the following recommendations are offered to the SPUC: ■ The SPUC should consider proceeding with supplying 3.0 MGD of water to the City of Savage. A water supply agreement with Savage could include a sunset clause of 3 - 5 years, at which time the condition of the Prairie du Chien - Jordan Aquifer could be reassessed for signs of over- development. The benefits from taking this action include the reduction of regional groundwater y stress on sensitive natural resources in the Savage area. ■ Over the next several years, groundwater levels in the SPUC municipal wells should be monitored, frequently and on a regular schedule during both static and pumping conditions to determine whether the Prairie du Chien - Jordan Aquifer elevations are decreasing over time. This data can be used to identify long -term trends in the aquifer's condition and could be utilized to better calibrate the groundwater flow model. ■ The groundwater flow model should be improved through further refinement and better calibration. Improvements to the model will make it a more useful tool in evaluating and assessing potential impacts from groundwater withdrawal. Additional hydrogeologic data should be collected in the vicinity of Thole-Schneider- O'Dowd Lakes and the local buried bedrock valleys to better refine the groundwater flow model and improve its accuracy. Specifically, information from existing and future wells, in addition to geotechnical data may assist in replicating these features in the model and in — determining whether the Prairie du Chien - Jordan Aquifer in these localities is being recharged, or whether these areas are groundwater discharge areas. The groundwater flow model could also be better refined through the collection, evaluation, and incorporation of streamflow and river discharge data. By collecting actual streamflow and river discharge data and simulating these conditions in the model, the accuracy of the model should improve and become a more useful tool in evaluating and assessing various future conditions. — The model should be updated to replicate a resistance to groundwater flow between the Prairie du Chien Group and the Jordan Sandstone. Aquifer Sustainability Study A- SHPUC0501.00 Shakopee Public Utilities • As additional municipal wells are constructed, the SPUC should continue to collect hydrogeologic data through comprehensive aquifer pumping tests. This data can supplement existing data and could be useful in refining the groundwater flow model. • The SPUC should continue to cooperate and collaborate with the Southwest Metro Ground Water Group to identify strategies and best management practices to minimize the groundwater use and pumping development pressure on the aquifer. • The SPUC may want to consider conducting a more thorough groundwater study on how the local and regional sensitive natural resources (e.g. springs, trout streams, and calcareous fens), dependent upon groundwater discharge, may be affected under future, increased pumping conditions. Aquifer Sustainability Study A- SHPUC0501.00 Shakopee Public Utilities