SITE INFORMATION AND CORRESPONDENCE

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equation,the pumping rate (Q)required for a desired vertical flow. Thus information from other lines of width of capture(W) can be estimated. The pumping evidence may be required. rate required for a given capture width must generally be higher than estimated by this equation to account for Ground Water Elevation Pairs. In some cases, pairs recharge and uncertainties in the other parameters. of ground water elevation measurements on either side Similarly, actual capture width for a specified pumping of a boundary can be used to demonstrate inward flow rate will typically be less than estimated by this relative to that boundary. An example might be equation for the same reasons. ground water elevation measurements on either side of property boundary or on either side of a slurry wall. This approach for idealized situations can and should Another example would be stage measured in a creek be used as a rudimentary analysis of ground water flow relative to the ground water elevation in the aquifer at a site. However, the simplifying assumptions and immediately adjacent to the creek. A higher creek resulting limitations should be understood and stage indicates no discharge from the aquifer to the specified. The limitations of this approach strongly creek. Because flow between the creek and aquifer can indicate a need for considering additional lines of change magnitude and even direction with changes in evidence for evaluating capture. precipitation and recharge, frequent measurements from these locations may be required. Ground water Potentionretr•ic Surface Maps. Ground water elevation elevation pairs from different levels of the aquifer can measurements can be used to create potentiometric also be used to verify vertical gradients that are surface maps, from which ground water flowlines and indicative of capture. Generally, it is important to the capture zone can be interpreted. Unfortunately,the exclude ground water elevations from active pumping number of ground water elevation measurements wells and to consider recent recharge events. typically available is not sufficient to unambiguously interpret capture. It is important to exclude ground Sentinel Wells. If capture is adequate,monitoring water elevation data from active pumping wells when wells downgradient of the extraction system(i.e., constructing potentiometric surface maps because they sentinel wells)can be monitored over time as follows: are influenced by well losses and are not representative of aquifer conditions. Note that when potentiometric Sentinel wells that are not currently impacted surface maps indicate capture with respect to horizontal by contaminants should remain without flow,capture may not be adequate with respect to impacts over time. Exhibit 4 Sentinel wells that are currently impacted by Width of Capture Zone and Flow Budget For contaminants should reach background levels over time. If concentrations decrease in these Very Simple Hydrogeologic Systems wells but remain over regulatory standards, Assumptions: capture provided by the extraction system is • one well likely inadequate. • single layer of constant thickness • homogeneous, isotropic aquifer Because ground water flow is slow, impacts at sentinel • no recharge from above or below wells may take years to appear, and concentration Q measurements over time at sentinel wells can become W= very costly. Interpretation may be ambiguous if the C X B X K X i sentinel wells are actually located within the zone of capture or if they are not in the correct locations to or detect uncaptured portions of a plume. Also, if the plume is not well delineated,portions of the plume Q= W x C x B x K x i may have previously migrated beyond the capture zone and the sentinel wells. These limitations of Q=extraction rate(gpm) sentinel wells emphasize the importance of using C=conversion factor(0.00518 gal/ftmin/day) multiple lines of evidence. For sites with fractured W=total width of capture zone upgmdient of the extraction system(ft) bedrock and/or highly heterogeneous conditions a B=saturated thickness(ft) greater density of sentinel monitoring points may be K=hydraulic conductivity(ft/day) merited due to the increased potential for preferential i=hydraulic gradient(ft/foot) pathways of contaminant migration. In addition,for sites with multiple layers or stratigraphic units with 5