Preliminary investigation for controlling the source of groundwater contamination in Phoenix Arizona, USA

摘要

In 1984, the Arizona Department of Environmental Quality (ADEQ) identified seven possible sources of groundwater , contamination in the East Central Phoenix (ECP) area. Water quality data indicated perchloroethylene (PCE) concentration level ranging from below 5 micrograms per liter (ug/1) to over 3000 ug/h. Three of 11 production wells with average capacity yield of over 900 gallons per minute (gpm) located in the areashowed detectable levels of PCE in excess of the Aquifer Water Quality Standard (AWQS). Additional site characterization helped define the extent and degree of contamination, thus qualifying the ECP to receiving cleanup funding from the Arizona State Water Quality Assurance Revolving Fund (WQARF). WQARF fund allocation is based on risk evaluation and eligibility ranking; it does not reflect the extent and level of remediation desired at any particular site or time frame for achieving plume control and complete aquifer restoration. Sites that rank high on ADEQ list receive priority consideration; low ranking sites remain unattended as groundwater contamination continues to spread, potentially exposing down gradient off site thirdparty facilities. Well owners and water providers in the area were therefore constrained from pumping their wells, as doing so would have the potential of mobilizing the plume, exacerbating the contamination problem. These stakeholders approached the ADEQ with a proposal to undertake an interim remedial action (IRA) to implement source-control of volatile organic compounds (VOC). The objective was to utilize shallow tray aerators to remove VOC mass from the aquifer, control migration of contaminant source and remove further threat of contamination. The agency agreed to fund the proposed IRA if there is evidence of a potential connection between the plume source and at least one nearby production well in the area. An investigation was initiated to assess potential plume-to-well connection, evaluate the feasibility of VOC source-control and possible plume capture. Preliminary results of a conceptual hydrogeologic and numerical groundwater flow modeling are presented. The model established 5 distinct Layers with varying hydraulic conductivity values. Steady state and transient flow conditions have yet to be fully calibrated for the upper layer aquifer units where depth-to-groundwater is about 55 feet below ground surface. However, particle traces calculated using MODPATH show rapid movement towards the well for particles reaching relatively deeper coarse-grained alluvium Layer #4. Potential for plume mobilization towards the well appears to exist.