Modeling Vadose Zone Processes during Land Application of Food‐Processing Waste Water in California's Central Valley

Abstract

Land application of food-processing waste water occurs throughout California's Central Valley and may be degrading local ground water quality, primarily by increasing salinity and nitrogen levels. Natural attenuation is considered a treatment strategy for the waste, which often contains elevated levels of easily degradable organic carbon. Several key biogeochemical processes in the vadose zone alter the characteristics of the waste water before it reaches the ground water table, including microbial degradation, crop nutrient uptake, mineral precipitation, and ion exchange. This study used a process-based, multi-component reactive flow and transport model (MIN3P) to numerically simulate waste water migration in the vadose zone and to estimate its attenuation capacity. To address the high variability in site conditions and waste-stream characteristics, four food-processing industries were coupled with three site scenarios to simulate a range of land application outcomes. The simulations estimated that typically between 30 and 150% of the salt loading to the land surface reaches the ground water, resulting in dissolved solids concentrations up to sixteen times larger than the 500 mg L(-1) water quality objective. Site conditions, namely the ratio of hydraulic conductivity to the application rate, strongly influenced the amount of nitrate reaching the ground water, which ranged from zero to nine times the total loading applied. Rock-water interaction and nitrification explain salt and nitrate concentrations that exceed the levels present in the waste water. While source control remains the only method to prevent ground water degradation from saline wastes, proper site selection and waste application methods can reduce the risk of ground water degradation from nitrogen compounds.

Reactive transport codes for subsurface environmental simulation

“…For example, the code has been used by a number of researchers to simulate the generation and attenuation of acid rock drainage in mine waste (e.g., [202]), source zone natural attenuation of hydrocarbon spills in variably saturated media (e.g., [208,216]), land application of foodprocessing waste water [217], and biogeochemical cycling of silicon in forest soils [218].…”

Solution of the MoMaS reactive transport benchmark with MIN3P—model formulation and simulation results

“…To maximize versatility, the model formulation includes a generalized framework for kinetically controlled reactions, which can be specified through a database together with equilibrium processes. The transport and reaction modules of the code have been extensively verified and the code has previously been used for various applications including the generation and attenuation of acid mine drainage [e.g., 2, 3], biodegradation of organics involving multiple electron acceptors [21,42], groundwater remediation using permeable reactive barriers [23], element cycling in forest soils [9], land application of food processing waste water [24], and for the evaluation of redox stability in deep crystalline rock formations considered for deep geologic repositories for nuclear waste [33]. The code has also served as a platform for additional developments including gas exsolution, entrapment, and release [1], and multicomponent advective and diffusive gas transport [25].…”

Environmental Impacts and Sustainability of Degraded Water Reuse

“…Th e main concerns with land application of this wastewater are associated with increased loads of salinity and nitrate, and to a lesser degree ammonia (NH 3 ), Fe, Mn, SO 4 , and BOD. Miller et al (2008) used the multi-component reactive fl ow and transport model MIN3P (Mayer et al, 2002) to simulate movement of various chemical constituents, particularly labile OC, salinity, and N, found in land application of food-processing wastewater for the California Central Valley. Th eir objective was to evaluate the transport and attenuation of these waste constituents within the vadose zone, and to provide estimates of OC, salinity, and N loads in the Central Valley.…”