Projects Southwest


91ST AVENUE WWTP

Project Details
CLIENT   Greeley and Hansen, LLC
DATES  Aug. 2010 – Mar. 2011
TASKS  Investigation of Alert Level (AL) Exceedance for Nitrate

DESCRIPTION
Matrix was retained by Greeley and Hansen to conduct a hydrogeologic investigation in an attempt to determine the cause of AL exceedances for nitrate in groundwater samples at the 91st Avenue Wastewater Treatment Plant (WWTP). Matrix reviewed all available data and background information concerning operation of the WWTP and groundwater monitoring. Because historical data for plant effluent discharges show Nitrate-N concentration values below 10 mg/L, it was concluded that there is no connection between plant effluent discharges and the high nitrate levels detected in the groundwater monitoring wells.

Following extensive review of groundwater elevation data from 2003 to the present, it became apparent that the periods of high nitrate concentrations in groundwater samples appeared to coincide with periods of recharge related to flows in the Salt River channel. It is Matrix’s opinion that these periodic flooding events, which ultimately lead to higher groundwater elevations, are mobilizing nitrate in the vadose zone resulting in the high nitrate levels.

Matrix prepared a hypothetical groundwater flow model for the area using local monitor well and dewatering well data. Results of the model indicate that on-site dewatering activities do not appear to be changing the groundwater gradient to the point of causing nitrate contaminated groundwater to migrate onto the site. Matrix and Greeley and Hansen presented the results to the City of Phoenix for their review and approval.