Quantitative assessment of the shallow ground-water flow system associated with Connetquot Brook, Long Island, New York

Abstract

Streamflow on Long Island is derived principally from shallow ground water that flows above the deeper regional flow system. The movement of shallow ground water was studied during 1975-82 at Connetquot Brook, an undisturbed stream in Connetquot River State Park, in south-central Long Island, New York. The investigation encompassed (1) field studies of streamflow, ground-water levels, and age of water as indicated by tritium and dissolved-oxygen concentrations and (2) numerical simulation of the shallow flow system to evaluate the hydraulic factors that govern the direction of shallow ground-water flow near and beneath the stream. Analysis of water-level data indicates that ground water flows essentially horizontally throughout the drainage basin except near and beneath the stream, where it moves vertically upward toward the stream discharge boundary. Water levels in wells driven directly into the streambed and into the streambank at three sites were 1 to 2 feet higher than stream stage in the first 5 feet of penetration. Increases in head, which were detected to depths of 30 feet beneath the streambed, indicate upward movement of water above that depth. Hydraulic conductivity of the streambed sediments was calculated from head gradients in the streambed and from measured stream seepage; values were between 11 and 15 feet per day. Water samples from selected wells were analyzed for dissolved-oxygen and tritium concentrations to determine the relative age of the water in an attempt to locate the bottom boundary of the shallow flow system. Dissolved oxygen showed no pattern, but tritium concentrations about 1,000 feet from the stream were lower than those near the stream. The tritium concentrations indicate that the lower flow boundary was between 45 and 100 feet below the water table. A two-dimensional cross-sectional flow model of the shallow flow system was developed. The near-stream model response compared well with field data when the streambed discharge boundary was simulated as a uniform leaky bed. A systematic sensitivity analysis was done to determine which factors have the greatest influence on hydraulic head in the system. Ten dimensional parameters that describe the important aspects of the flow system were combined into a series of dimensionless parameters to simplify analysis. Results indicate that (1) streambed factors (width and hydraulic conductivity) are most influential upon heads near the stream, (2) factors representing thickness of the shallow flow system influence heads distant from the stream but have a negligible effect near the stream, and (3) factors that represent the quantity of water entering the system (recharge) influence the heads throughout the area. Field measurements of hydraulic head indicate that the thickness of the shallow flow system below the stream channel is about 30 feet. However, results of the sensitivity analysis indicate that the shallow system's thickness has a negligible effect on head distribution beneath the stream.