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Evaluate Extraction Well Performance becomes operationar Specific capacity should then be measured regularly and a trend line plotted. The Extraction wells should be monitored to determine if average extraction rate should also be compared to the they are in the most effective location, given potential design extraction rate. If a decrease in specific changes in the contaminant plume,and also to capacity or the extraction rate of more than 10%is determine if they are performing as expected. noted, well-rehabilitation is likely required. In some cases, a well-maintenance program may need to be Evaluate Concentration Trends at Extraction Wells implemented. Note that for a well operating at a constant rate, drawdown trends provide the same As contaminant mass is extracted from the subsurface, information as specific capacity trends. For wells the contaminant concentrations in extraction wells where ground water elevations cannot be measured should decrease unless a continuing source of due to restrictions in access,measurement of the contaminant exists or contaminant levels have reached extraction rates will have to suffice (i.e., decreasing a plateau. If the contaminant concentration in an rate over time may indicate fouling). extraction well, or nearby monitoring wells,has decreased to low levels, then it may be more effective Regional changes in the ground water elevations due to shut down that extraction well, relocate it, or . to drought, recharge,or off-site pumping should be reallocate ground water extraction to other wells. considered by reviewing ground water elevation trends However, shutting down a well may not be possible if in nearby monitoring wells. Regional increases in that well is required for capture of the contaminant ground water elevations could mask a decrease in plume. If a well is shut down, monitoring should specific capacity, and regional decreases in ground continue for some period of time to ensure that water elevations could falsely suggest a decrease in concentrations do not"rebound"due to desorption of specific capacity. contaminants from soil to ground water,diffusion of contaminants from tighter portions of the formation,or A general guide to well maintenance developed by the additional dissolved contamination from continuing U.S. Anny Corps of Engineers (USACE)can be found sources. Monitoring ground water concentrations in in USACE Engineering Pamphlet EP 1110-1-27. individual extraction wells on an annual or semi-annual basis is likely sufficient for the above analyses. If Applicable, Evaluate Injection Well or Infiltration Gallery Performance Pumping Rates and/or Specific Capacity Versus Time If a system discharges treated water to the subsurface Bacterial growth and chemical deposits can lead to through injection wells or an infiltration gallery,the fouling of extraction wells. If addressed properly performance of these points should be monitored to through a well-maintenance or well-rehabilitation ensure fouling does not limit the discharge capacity, program, fouling can usually be mitigated. If well and therefore the capacity of the entire P&T system. fouling is left unattended, however,it may reach a point where well rehabilitation is infeasible and the Many of the same procedures for monitoring well will need to be reinstalled. extraction wells can be used to monitoring injection points. The discharge rate and the increase in ground In general,fouling blocks the well screen and provides water elevation within the wells or galleries are resistance to water entering the well. As a result,the measured rather than the extraction rate and the water level in the well decreases until there is a drawdown. sufficient hydraulic gradient directing water into the well to balance that being extracted by the pump or Increases in the water levels within the reinjection until the water level in the well drops below the pump. points,given a constant discharge, indicate either Fouling may therefore occur with no noticeable change regional increases in ground water elevation or fouling in the extraction rate until the pump shuts down. of the injection wells. Regional increases in the ground water elevation can be confirmed or rejected The best indicator of well fouling is the specific by reviewing ground water elevation trends in nearby capacity of the well,which is the extraction rate monitoring wells. If increases are not due to regional divided by the drawdown(note that the ground water influences a rehabilitation program may need to be elevation under both static and pumping conditions is implemented, and the treatment system should be required to calculate drawdown). A baseline level of reviewed for options to minimize solids in the specific capacity should be recorded when the well first effluent. 11