Processes Affecting Agricultural Drainwater Quality and Organic Carbon Loads in California's Sacramento-San Joaquin Delta

Abstract

From 2000 to 2003 we quantified drain flow, drainand ground-water chemistry and hydrogeologic conditions on Twitchell Island in the Sacramento-San Joaquin Delta. The primary objective was to quantify processes affecting organic carbon concentrations and loads in agricultural drainage water. We collected physical and chemical data in southern and northern areas: TN and TS, respectively. Corn grew in both areas during the spring and summer. The peat soils in the TN area are more decomposed than those in the TS area. Results elucidate processes affecting drain flow and concentrations under varying hydrologic conditions. During May through November, groundwater flows from the permanently saturated zone to drainage ditches, and the resulting average drainagewater quality and dissolved organic carbon (DOC) concentration was similar to the groundwater; the median DOC loads in the TN and TS study areas ranged from 9 to 27 g C/ha-day. The major ion chemistry and stable isotope data confirmed that groundwater was the primary source of drainflow. In contrast, during December through April the drainwater is supplied from the shallow, variably saturated soilzone. The DOC concentrations, major-ion chemistry and stable isotope data indicate the shallow-zone water is partially evaporated and oxidized. Higher flows and DOC concentrations during these months result in higher median DOC loads, which ranged from 84 to 280 g C/ha-day.During December through April, increasing groundwater levels in the shallow peat layers and mobilization of organic carbon result in high drain flow and increased trihalomethane precursor concentrations and loads. On a per mass DOC basis, drain water collected during high flow periods is less likely to form THMs than during low flow periods. However, the high flows and subsequent high concentrations contribute to substantially higher trihalomethane precursor and DOC loads. KEYWORDSagricultural drainage, organic carbon, drinking water quality, disinfection byproducts, organic soils

Organic Carbon and Disinfection Byproduct Precursor Loads from a Constructed, Non-Tidal Wetland in California's Sacramento-San Joaquin Delta

“…Because DOC concentrations in shallow porewaters were so much higher than in deep pore-waters, directional flow from the shallow to deep soil layer would have resulted in pore-waters with similar DOC compositional characteristics, but this was not the case. DOC from shallow soil pore-waters of agricultural fields nearly always had lower HPoA and greater TPiA percentages, than DOC from pore-waters in deeper soils (Deverel et al 2007; Figure 6d). The similarity in DOC quality of wetland soil layers and soil layers of drained fields indicates that pre-inundation soil characteristics dominantly affect the current pore-water DOC character, despite six years of permanent flooding figure 6D, the range of DOC fractionation results from pore-waters in shallow and deep soil layers in Twitchell Island agricultural fields are provided for comparison (Deverel et al 2007).…”

“…Groundwater flow directions are primarily lateral from the pond to the adjacent drainage ditch, and upward from the sand and silt layers into the organic soils. The broader hydrogeologic framework for the study area is described in greater detail in Burow et al (2005), Gamble et al (2003), and Deverel et al (2007 …”

“…Pier K never received sufficient input of well-oxygenated inlet water to replenish dissolved oxygen in the water column. It is likely that these flow conditions resulted in a much longer residence time for water in the vicinity of Pier K, compared to water in the vicinity of Pier H figure 6D, the range of DOC fractionation results from pore-waters in shallow and deep soil layers in Twitchell Island agricultural fields are provided for comparison (Deverel et al 2007). …”

“…We determine a water budget for the wetland, calculate net loads of organic carbon exported from the wetland, and estimate the change in these loads as the wetland matures. We then compare net organic carbon loads from the wetland to net organic carbon loads determined for agricultural fields on Twitchell Island by Deverel et al (2007). Finally, we assess the potential effects of wetland restoration on water quality of peat island drainage waters.…”

Factors and Processes Affecting Levee System Vulnerability

“…Higher percentages are associated with winter drainage. Deverel et al (2007) demonstrated that oxidation of soil organic matter and subsequent mobilization of DOC to drainage ditches are the primary mechanisms that lead to DOC export from subsided Delta islands. Oxidation of soil organic matter in drained agriculture also results in losses of carbon dioxide that range from 15 to 20 tons carbon dioxide per ha (e.g., Knox et al 2015), which could be stopped or greatly reduced if islands are converted to rice and permanently-flooded carbon sequestration wetlands (Knox et al 2015) or tidal wetlands.…”