Archive for October, 2012

Monitoring Private Wells as an EIR Mitigation Measure for Municipal Wells

Thursday, October 11th, 2012

Cameron Tana presented this at the CA-NV AWWA Annual Fall Conference October 11, 2012

Monitoring Private Wells as an EIR Mitigation Measure for Municipal Wells

Cameron Tana1, Georgina King2 , Kelly White3, and Taj Dufour4

1 HydroMetrics Water Resources Inc., Potomac, MD
2 HydroMetrics Water Resources Inc., Oakland, CA
3 Environmental Science Associates, San Francisco, CA
4 Soquel Creek Water District, Soquel, CA

In order to meet requirements of the California Environmental Quality Act (CEQA), Soquel Creek Water District (SqCWD) assessed potential impacts from operating up to five proposed production wells in its Well Master Plan Environmental Impact Report (EIR). One of the impacts assessed was the potential for pumping from the proposed wells to negatively affect nearby non-SqCWD wells, including private wells.

The Well Master Plan EIR considered pumping impacts on private wells to be significant if:

1. Drawdown from the proposed production wells would be expected to lower groundwater levels below the top of the private well screen, thereby increasing the risk of physical damage to the well; or
2. Drawdown from the proposed production wells would be expected to cause a loss of yield in the nearby private well such that its water quantity or quality is rendered incapable of meeting its historically measured production levels.

An analytical model indicated that the proposed production wells would not drop water levels below the average top-of-screen depth of nearby private well screens and would not cause a significant decline in well yield of the average nearby pumping well. The average top-of-screen depth is an appropriate benchmark for the analysis because it would be unreasonable for the shallowest well or a marginally performing well to constrain the use of basin supply by all users. However, to account for the possibility of adverse impacts to shallower non-average private wells, or to private wells for which information is not available, the EIR conservatively determined pumping impacts to nearby private wells to be potentially significant. To reduce these potential impacts to a less than significant level, SqCWD is implementing a voluntary monitoring and mitigation program for private well owners.

The program involves installing flow meters and groundwater level loggers in the private wells of owners that enroll in the program. The equipment is installed at least 6 months prior to the planned start-up of the nearby proposed production well to collect baseline data. Data collected after the proposed production well is online is used to evaluate whether pumping of the District well has had a restrictive effect on the private well.

SqCWD developed a waiver agreement for private well owners defining the terms and conditions of the program. SqCWD has completed enrollment and equipment installation for 13 private wells near the first of the proposed production wells that will come online. Once enrollment was completed, a field inspection of the private wellhead was completed to measure groundwater levels and collect information to plan installation of equipment. Installation of flow meters was typically straightforward. However, well seals with access ports too small to accommodate the groundwater level loggers were replaced and PVC pipes to protect the loggers were installed. The private wells were disinfected and dechlorinated before returning the wells to service. The loggers were installed at a later date. The presentation will outline information useful for the private well monitoring program, describe and show examples of implementation challenges and summarize lessons for improving the program.

Applications of Precipitation-Runoff Modeling System (PRMS) in Groundwater Studies

Thursday, October 4th, 2012

Georgina King gave this presentation at the Groundwater Resources Association of California 21st Annual Conference and Meeting October 4, 2012.

Applications of Precipitation-Runoff Modeling System (PRMS) in Groundwater Studies

Georgina King, PG, CHg, and Cameron Tana, PE

Water purveyors in the Soquel-Aptos basin, Santa Cruz County, California need recharge estimates to assist them making groundwater management decisions. Specific decisions are related to pumping curtailments during low rainfall periods, planning for recharge impacts due to future land use changes, and effects of shifting pumping closer to recharge areas. The USGS’s Precipitation-Runoff Modeling System (PRMS) was selected as the tool to determine the rainfall-recharge relationship for the basin, and thus be used to estimate the amount of recharge likely from available rainfall data. The model can also be used to identify or map areas of recharge.

PRMS simulates streamflow from precipitation, after evapotranspiration, and groundwater recharge are accounted for. Model input is daily climatic data: precipitation and temperature. Hydrologic and physical characteristics are assigned to the model area. Calibration using average monthly solar radiation, potential evapotranspiration, and daily streamflow as targets improved model credibility.

The deep recharge results from the PRMS model are also being used as the recharge input to a MODFLOW model for Central Water District, which pumps from the Aromas area of the Soquel-Aptos basin. The MODFLOW model will be used to evaluate shifting pumping inland closer to recharge areas in order to address water quality issues. MODFLOW estimates of stream percolation will be used to evaluate the uncertainty introduced by using PRMS and MODFLOW in sequence as opposed to coupling PRMS and MODFLOW in a GSFLOW model.