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Project Title: Hydrogeology and Groundwater/Surface-Water Interaction in and around Dakota County, Minnesota
Project Number: NQ00CM300 - MN 241
Project Chief: Cowdery, Timothy K.
Project Start Date: October 2006
Project End Date: ongoing
Cooperator: Dakota County Environmental Management

Statement of Problem:
Dakota County water managers need detailed and accurate groundwater information and understanding to make scientifically sound resource management decisions. Questions about the adequacy of groundwater supplies through the year 2030 have arisen because of anticipated increases in groundwater use and possible degradation of groundwater quality. Many water problems in the county involve the interaction of ground water and surface water. Dakota County’s ongoing ambient groundwater-quality monitoring program has identified nutrients, pesticides, and pesticides degradates in domestic and municipal water supplies. Tests have detected increasing nitrate concentrations over the past 10 years, particularly in the southeastern part of the County where glacial deposits are relatively thin and the bedrock aquifers more vulnerable to contamination. Concentrations of anthropogenic chemicals have exceeded Federal and State health limits in some domestic wells. Seventy-eight percent of the wells sampled contained at least one detectable pesticide. Although the state of groundwater quality in Dakota County is becoming clearer through this sampling, the current understanding of the groundwater system does not adequately explain the water-quality patterns observed. Existing groundwater models do not adequately explain the movement of water and associated anthropogenic chemicals found in the County sampling program. To begin the process of providing answers to questions posed by the demands on groundwater resources, models must incorporate detailed hydrostratigraphy, must be model-grid independent, and must be capable of transient simulations,and solute transport.

Objectives:
Gain a better understand the hydrogeology of Dakota County with emphasis on the role of groundwater and surface-water interaction, especially in the area of the Vermillion River. This understanding will be used by the cooperator to evaluate how best to allocate groundwater resources to meet increasing water demand while balancing these demands with environmental water needs tounderstand how patterns of recharge, groundwater flow, groundwater/surface-water interaction, groundwater use, and land use produce patterns of groundwater quality.

Relevance and Impact:
This study will begin to provide county managers with the data and understanding they need to manage the groundwater resources of the county through a time of changing land use, and increasing groundwater withdrawals. This study will allow managers to make more educated management decisions, helping to ensure greater protection of water supplies and groundwater quality. This study will increase understanding of water movement and solute transport in the County, the sources of anthropogenic chemicals in ground water, and the processes controlling the presence of these chemicals in domestic and municipal groundwater supplies. The water-quality data base produced for this study will be a valuable organizational and analysis tool for existing data and that collected in the future.

Progress During FY11:
The report modeling draft was submitted to colleague review. This report was tabled because of calibration and sensitivity analysis work added to the model. A detailed workplan for modeling efforts this fiscal year was completed and accompanied the 2011 agreement. Calibration flow data sets were prepared and incorporated into the model. These data included measured flows along the Vermillion River and Metro Model II estimated flows along other rivers. These flows were distributed among the finer-scale hydrography incorporated in this model. Minor layer errors in pumping data were corrected. The model was parameterized to allow for automated estimation and sensitivity analysis. The model was ported to a 64-bit, multiprocessor version of GMS successfully. This resulted in a decrease in model run time of about 2-3 fold. Parameter estimation and sensitivity analyses were computed. The parameterized model did not match the results of the unrameterized model. The software manufacturer has confirmed this problem and has confirmed that the parameterization is correctly done. They are trying to identify and resolve this problem.

Statement of Work FY12:
Parameter estimation and sensitivity analysis of the model will be completed. Discussions will proceed about how to ensure that a modeling report incorporating this new work and relevant to Dakota County information needs is produced. Water-quality samples will be quality-assured and incorporated into the USGS NWIS data base. A detailed workplan with deliverables and due dates will be written for FY 2012. We will continue to discuss our future role in the County's modeling needs and the possibility of a Vermillion River GW/SW HEM project.