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Land application of wastewater can be a cost-effective and environmentally responsible wastewater disposal method. To evaluate the impact of wastewater land application on groundwater resources, groundwater samples must be collected and analyzed. Suction lysimeters are a low-cost alternative to monitoring wells for measuring groundwater quality. Lysimeters can be used to collect groundwater from the unsaturated vadose zone, before the application area percolate mixes with groundwater from the upper aquifer.

Land application is used to dispose of wastewater generated at a truckwash facility in northern California, where tanker trucks hauling food-grade products (e.g., wine, molasses, vegetable oil) are rinsed and sanitized. Three lysimeters (two in walnut orchards and one in pasture) are used to monitor groundwater conditions at the truckwash land application facility. Lysimeter data collected from 1999 to 2001 were used to evaluate the impact of wastewater land application on groundwater quality, and to determine the effectiveness of the lysimeters in monitoring vadose zone groundwater quality.

Based on daily flow records and local meteorological data, a water balance was conducted to estimate the monthly volume of percolate leaching from each field. Using water quality data for the applied wastewater and supplementary irrigation well water, the monthly total nitrogen (TN) load for each field was calculated. Crop nitrogen uptake values were then used to estimate percolate TN concentrations for each field (annual averages of 0.0 to 3.0 mg/L). Lysimeter samples were collected and analyzed quarterly for TN (annual averages of 0.1 to 8.2 mg/L). The annual average lysimeter TN values compared well to the estimated TN values. Based on the results of the study, suction lysimeters may be relied upon to provide representative vadose zone groundwater quality data.