Using resistivity arrays to monitor groundwater impacts near runoff holding ponds

Authors: Eigenberg, Roger; Woodbury, Bryan

Submitted to: Journal of Environmental & Engineering Geophysics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/27/2012
Publication Date: 6/1/2012
Citation: Eigenberg, R.A., Woodbury, B.L. 2012. Using resistivity arrays to monitor groundwater impacts near runoff holding ponds. Journal of Environmental & Engineering Geophysics. 17(2):103-112.

Interpretive Summary: Cattle feedyard operators use holding ponds to contain rainfall runoff from the pen surfaces. Water in these ponds contains contaminants that should be kept away from underground water supplies. Most of the ponds are constructed and managed to limit the possibility of the pond’s contents leaking; however, some leakage does occur. Traditional methods used for monitoring leakage from these ponds require the installation of numerous monitoring wells to allow for removal and analysis of water samples. These monitoring wells are expensive to install, the measurements are costly and the information received difficult to clearly interpret. This paper describes a completely different method for monitoring leakage from a holding pond. A set of probes were installed next to the side of two different runoff storage ponds a foot underground to protect the equipment and allow for routine vegetation management. Each probe was connected to a central reader located above ground to measure the amount of contaminants leaving the pond to the surrounding soils. This new method allows for automated and continuous monitoring at a number of depths and locations around the pond. The first pond had soil types that limited water leaving to the surrounding soil. The new monitoring system confirmed very little seepage from this pond. The second pond was in a different soil that allows more water to seep to the surrounding soil. The new monitoring system confirmed that under certain conditions water seeped from the pond to the surrounding soil. The operators of the second pond have used this new information to manage this pond differently to limit seepage from this pond. The results of this study are encouraging and more work is planned to further validate the effectiveness of the new monitoring system.

Technical Abstract: Mineral and organic salts from beef manure contained in precipitation runoff from feedyard pen surfaces can alter the conductivity properties of soil and water receiving it. Typically, holding ponds are constructed to control runoff from concentrated animal feeding operations. The integrity of these holding ponds has come under increased scrutiny since leakage has the potential to affect soil and groundwater quality. Traditionally, ponds are monitored by installing monitoring wells at key locations to evaluate the impact of these ponds on the environment. These monitoring wells are expensive and subject to ambiguous interpretation. A subsurface resistivity array was installed at a beef cattle feedyard located at the U.S Meat Animal Research Center, Clay Center, Nebraska (Feedyard A) and at a cattle feeding cooperator site (Feedyard B). Array probes were permanently installed at Feedyard A (16 probes spaced 6.1m apart at a depth of 30 cm) and at Feedyard B (32 probes spaced 3.05m apart at a depth of 50 cm). Weekly readings from each site were evaluated to monitor the stability of the zone of hydration near the pond. The low hydraulic conductivity soils at Feedyard A provided a very quiescent environmental system to evaluate the resistivity array’s inherent measurement stability. Seasonal changes could easily be accounted for by variation in seasonal soil temperatures. The Feedyard B site was typified by coarse textured parent material that had high hydraulic conductivity properties. This site experienced dynamic changes week to week and throughout the season. The resistivity array system was able to adequately measure these dynamics. Additional analysis using difference maps improved the illustration of the resulting dynamic. The results of this study indicate that the resistivity array system has the potential to improve monitoring of runoff holding ponds and warrants additional validation.