HETEROGENEITY EFFECTS ON PUMPED WELL WATER SAMPLES: USE OF PURGE ANALYZER TOOL (PAT) TO IDENTIFY HYDROGEOLOGIC FACTORS ON TIME-VARYING CAPTURE OF GROUNDWATER

In-well time of travel of well water to pump intake position is a key factor affecting sample water chemistry prior to achieving steady-state transport. Steady-state transport is defined as the time needed to achieve complete capture of inflowing groundwater along the entire well opening to the pump intake position. Prior to steady-state transport, pumped water consists of a mixture of ambient well water or pre-existing screen water and induced inflow of groundwater from the aquifer. Although steady-state flow conditions of the pumped well may be achieved relatively quickly during sampling based on hydraulic properties of the aquifer, water-level stability and pump rates, the duration of transient transport (pre-steady state transport) may take substantially longer periods to overcome and exceed more than three times the equivalent volume of well water to be purged (a traditional purge target). A major control on in-well transient transport is the heterogeneity of the aquifer relative to the position of the pump intake. Specifically, the location of the pump intake relative to the degree of heterogeneity of the aquifer and the position of any high hydraulic conductivity layers intersecting the well. For example, in highly heterogeneous aquifers such as fractured rock, the time needed to achieve steady-state transport and the rate of capture of inflowing groundwater is substantially different whether water-bearing fractures are proximal or distal to the pump intake position. A Visual Basic for Applications (VBA) analytical model named the Purge Analyzer Tool (PAT) has been developed to compute rates of in-well time of travel for observation wells undergoing purge and sample methods. The PAT employs a water-budget approach to solve a modified version of the Dupuit-Thiem equation. It then solves for time of travel from outside the pump intake zone (called the mixing zone), assuming piston transport of groundwater flow. Use of PAT allows for determination of the time needed to achieve steady-state transport, likely capture intervals over time, and the ability to assess the representativeness of groundwater samples.