AQUIFER MAPPING INTEGRAL TO WELLFIELD PROTECTION EFFORTS

 

By Kenneth Taylor, Senior Hydrogeologist, and Michael Manolakas, Hydrogeologist,
Trumbull, Connecticut office

Connecticut was one of the first states to enact aquifer mapping laws. Section 22A-354b-1 of the Connecticut General Statutes requires all public water providers serving more than 1,000 residents to map recharge areas for supply wells located in stratified drift aquifers. Pursuant to the federal directive requiring all states to establish wellhead protection plans, these "level A" maps will eventually become the basis for new land use regulations aimed at restricting development near wellfields.
    LBG has been retained by numerous clients seeking to comply with the aquifer mapping laws. The firm uses computer modeling to delineate the areas of influence, contribution and recharge around wellfields with a high degree of accuracy. Accuracy is critical not only to formulating effective land use regulations, but also to defending those regulations in court. It is best achieved by mixing small parts art and intuition with large parts data and hard science.
    A recent project completed for the Bridgeport Hydraulic Company (BHC) illustrates how conditions in constant flux such as groundwater flow can be translated into static but useful maps. BHC commissioned LBG to produce level A mapping for its Hamill Well Field in Litchfield, Connecticut. The wellfield is located in the Upper Housatonic River Basin and is surrounded by residential properties, a nature preserve and a cemetery. Its three production wells are authorized to withdraw a total of .388 mgd (million gallons per day).

Collecting Data
The study was initiated with the gathering of existing wellfield documentation, including pumping test results, water commission well records, state diversion records and previously published maps and data. A data collection plan was then developed and submitted to the Connecticut Department of Environmental Protection. Implementation of the plan involved installing additional monitoring wells, conducting pumping tests to identify aquifer characteristics and performing stream gauging to determine the effect of surface water bodies on the aquifer.
    Once the data was in, aquifer parameters were fed into MODFLOW, a 3-D numerical flow model developed by the U.S. Geological Service, to create a ground-water flow model. The model generated by MODFLOW had to be carefully calibrated to accurately simulate field conditions. This is where the art came in. The large amount of data collected made the model more usable, but also more difficult to calibrate. The model was also complicated by the existence of a silt clay confining layer within the primarily sand and gravel aquifer, which divided part of the aquifer in half. A third layer had to be added to the model to represent this confining unit.
    Aquifer parameters were tweaked and nudged. The process involved continually trying new combinations until a good match was found between model and field conditions. Once the ground-water heads were matched the system was stressed with pumping tests, with the parameters adjusted again until changes in the model mirrored changes in the real world.

Using Simulations
    Once this critical goal had been achieved simulations were run to determine the zone of influence. First the model was run with no wells pumping, for a steady state condition. Using the results of the steady-state run, the model was run with drought conditions for 180 days. Then the steady state was run with added pumping at an average, historical rate. The results of the steady state average pumping simulation were used as the starting point for a 180 day drought run with maximum permitted pumping levels. The zone of influence was determined by subtracting the no-well condition from the maximum pumping condition: anywhere drawdown exceeded half a foot was within the zone of influence.
    The area within the zone of influence from which water travels directly into a well is called the area of contribution. For the BHC project this area was delineated by placing particles in the model and observing where they went and what caused them to move, taking into account the effects of rivers and other factors. The area of contribution is restricted to the modeled area; the recharge area, on the other hand, extends beyond the model. All water within the drainage basin that flows directly into the capture zone is direct recharge. Water that rolls into a river, swamp or other surface water body before entering the area of contribution is indirect recharge. The combined sources of direct and indirect recharge constitute the aquifer recharge area.

Getting Regulatory Approval
When all the areas specified in the statutes had been delineated the data, methodology and results were presented in an extensive report which was sent to the Connecticut DEP for approval. This report explained in detail how the project data was collected and analyzed, how the model was built and calibrated, and what match was achieved between field conditions and the numerical model. It also included a map of the four delineated zones.
    The level A wellhead protected area is currently waiting for final approval from the state. Under Connecticut law, the report will then be submitted to the Town of Litchfield for use in developing new land use regulations. The Town is charged with the task of protecting the delineated direct recharge area from potentially polluting industries.
    The Connecticut statutes include a provision for appealing whatever land use regulations finally emerge. Since the regulations will be based in large part on the Level A mapping study, the outcome of any appeal will depend largely on the strength of our conclusions. Thanks to the extensive data that was collected and the substantial calibration effort that was expended on the project, the client and the town can feel confident that those conclusions are technically defensible.