Maryland Department of Natural Resources

Reports

Optimization of ground-water withdrawals in Anne Arundel County, Maryland, from the Upper Patapsco, Lower Patapsco, and Patuxent aquifers projected through 2044


2007, Andreasen, D.C.

Report of Investigations 77


Abstract

Withdrawals from public-supply wells operated by the Anne Arundel County Department of Public Works on average totaled approximately 26 million gallons per day in 2002. Of that amount 2.2, 17.2, and 6.2 million gallons per day were pumped from the Upper Patapsco, Lower Patapsco, and Patuxent aquifers, respectively. The Anne Arundel County Department of Public Works operates five major well fields in those aquifers located in four pressure zones in the central and northern portions of the county. In response to pumping, water levels in Anne Arundel County have declined to as much as 90 feet below sea level. Currently there is adequate available drawdown to sustain the withdrawals. Average-day water demand, however, is projected to increase nearly three-fold to 73 million gallons per day by 2040, with an estimated maximum-day withdrawal of 140 million gallons per day. An increase of that magnitude could cause significant drawdown resulting in water levels falling below the regulatory management level in some areas, well operational problems, increased pumping costs, and reduced stream baseflow. To minimize the regional drawdown effect of the increased demand, withdrawals from Anne Arundel County’s public-supply wells were optimized using a numerical, three-dimensional ground-water-flow model (MODFLOW code) constructed for this study in conjunction with an optimization algorithm (MODMAN and SuperLINDO codes) for the period 2005 to 2044. Water demands for Anne Arundel County’s well fields were based on the 2003 Comprehensive Water Strategic Plan prepared by O’Brien and Gere. The simulation period was extended to 2044 in order to model the effects of the projected 2040 water demand. Additionally, the effects of projected maximum-day withdrawals were evaluated.

Optimized Withdrawals from Anne Arundel County Department of Public Works Supply Wells to Meet Projected 2040 Average-Day Demand of 73 Million Gallons Per Day (pgs. 56 to 69)

The projected average-day demand, optimized to reduce regional drawdown, could be withdrawn without causing water levels to fall below the management level near the well fields. However, the increased withdrawals resulted in relatively deep water levels that increased pumping lift, which would lead to greater energy costs. In addition, the increased withdrawals could eventually result in some reductions in baseflow to streams within the recharge (outcrop) areas of the pumped aquifers.

Effects of Projected 2040 Maximum-Day Withdrawals from Anne Arundel County Department of Public Works Supply Wells (pgs. 69 to 71)

While ground-water supply sustainability is more related to the response of the aquifer system to long-term withdrawals, the effects of short-term withdrawals are also important considerations especially as it pertains to well operations. Withdrawals during the day of maximum use – projected to increase to 140 million gallons per day by 2040 – might cause significant drawdown resulting in isolated well-operational problems.