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

Steven J Deverel,Mark R Finlay,David A Leighton

doi:10.15447/sfews.2007v5iss2art2

Abstract

From 2000 to 2003 we quantified drain flow, drain-and 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 drainage-water 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 soil-zone. The DOC concentrations, major-ion chemistry, and stable isotope data indicate that 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.

Highlights

The Sacramento–San Joaquin Delta is the hub of California’s water supply system
Understanding the factors and processes contributing to dissolved organic carbon (DOC) and THM formation potential (THMFP) on Delta islands is pivotal to developing management strategies to minimize the THMFP in exported drinking water
We focused data-collection efforts in two areas on Twitchell Island (Figure 1); the northwestern area (TN) which includes the area studied by Fujii and others (1998), and the southeastern area (TS)

Summary

Introduction

The Sacramento–San Joaquin Delta is the hub of California’s water supply system. Drinking water for over 22 million Californians flows through the Delta channels from water-rich northern California to waterpoor southern California. Dissolved organic carbon (DOC) concentrations in Delta export waters can be problematically high due to potential formation of disinfection byproducts (DBPs) such as trihalomethanes (THMs) during the drinking water treatment process. Amy et al (1990) estimated that agricultural drainage from Delta island organic soils contribute 20% of the THM formation potential (THMFP) at Delta export pumps. Understanding the factors and processes contributing to DOC and THMFP on Delta islands is pivotal to developing management strategies to minimize the THMFP in exported drinking water. Aerobic oxidation of organic carbon, the primary cause of subsidence (Deverel and Rojstaczer 1996), began in the late 1800s as the nutrient-rich soils were cleared and dewatered for agriculture. By decreasing the land mass supporting levees, subsidence contributes to levee instability, failure, and water supply vulnerability (Procopovitch 1985)

Methods

Discussion

Conclusion