Mass Balance Analyses of Flushing, Evaporation, Infiltration, and Biological Activity in California Park Lakes, Butte County, California

California Park Lakes are retained by dams on Dead Horse Slough, an ephemeral stream network, which flows over volcanic/sedimentary rocks of the Tuscan Formation at the eastern margin of the Sacramento Valley, Butte County, California. Surface area of the lakes is approximately 0.2 km², maximum depth is 8 m, and approximate lake volume is 700,000 m³. Flow rate and depth measurements in Dead Horse Slough indicate that discharges associated with winter storms flush the lakes repeatedly. During dry summer months no surface water discharges from the lakes and water is pumped from the underlying aquifer to maintain the water level. Average pumping rates are approximately 1250 m³/day. Surface and storm drain runoff, mainly from nearby lawn irrigation, also supply water at fluctuating rates estimated to equal a fraction (e.g., 35 percent) of well water supply. Reported pan evaporation data indicate that summer evaporation from lake surfaces is 1000 to 1500 m³/day. Thus, the estimated evaporation rate approximately equals the well water and runoff supply rate. Periodic chemical analyses of water supplies and lake water have been conducted primarily to evaluate nutrient sources. Typical nitrate and chloride concentrations are 2 to 10 mg/liter, and typical phosphate concentrations are 0.2 to 1 mg/liter. Chloride concentrations in lake water during summer 2002 increased slightly corresponding closely to mass balance calculations for evaporative concentration. Infiltration (including leakage through the terminal dam) is small relative to evaporation and difficult to measure within the uncertainty of chloride measurements and water mass balance estimates. Lake concentrations of nitrate and phosphate are generally lower than source water concentrations consistent with biological consumption. Mass balance analyses of dissolved nutrients in lakes and water sources compared to chloride provide a measure of the rate of biological activity in the lakes, which is a water quality management issue. Characterization of flushing, evaporation, and biological activity in California Park Lakes using water and chemical mass balance analyses contributes to water quality management and provides constraints on infiltration into the Tuscan Formation, which is the primary regional aquifer for domestic water supplies.