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

Jacob A Fleck,Miranda S Fram,Roger Fujii

doi:10.15447/sfews.2007v5iss2art1

Abstract

Wetland restoration on peat islands in the Sacramento-San Joaquin Delta will change the quality of island drainage waters entering the Delta, a primary source of drinking water in California. Peat island drainage waters contain high concentrations of dissolved and particulate organic carbon (DOC and POC) and organic precursors to drinking water disinfection byproducts, such as trihalomethanes (THMs). We quantified the net loads of DOC, POC, and THM-precursors from a constructed subsidence mitigation wetland on Twitchell Island in the Delta to determine the change in drainage water quality that may be caused by conversion of agricultural land on peat islands to permanently flooded, non-tidal wetlands. Creation of permanently flooded wetlands halts oxidative loss of the peat soils and thereby may mitigate the extensive land-surface subsidence of the islands that threatens levee stability in the Delta. Net loads from the wetland were dominated by DOC flushed from the oxidized shallow peat soil layer by seepage flow out of the wetland. The permanently flooded conditions in the overlying wetland resulted in a gradual evolution to anaerobic conditions in the shallow soil layer and a concomitant decrease in the flow could be minimized by reducing the hydraulic gradient between the wetland and the adjacent drainage ditch. Estimates of net loads from the wetland assuming efflux of surface water only were comparable in magnitude to net loads from nearby agricultural fields, but the wetland and agricultural net loads had opposite seasonal variations. Wetland surface water net loads of DOC, POC, and THM-precursors were lower during the winter months when the greatest amounts of water are available for diversion from the Delta to drinking water reservoirs.

Highlights

For over a century, subsidence of peat soils has led to an increasing need for subsurface drainage on over 60 islands and tracts in California’s Sacramento-San Joaquin Delta
The results of this study indicate that the net loads of dissolved organic carbon (DOC), POC, and THM-precursors from a permanently flooded wetland supporting dense emergent vegetation can be similar in magnitude to the net loads produced by agricultural management of similar areas with peat soils (Figure 7)
The construction of the non-tidal wetland on Twitchell Island resulted in higher loads of DOC and THM precursors to Delta waterways

Summary

Introduction

Subsidence of peat soils has led to an increasing need for subsurface drainage on over 60 islands and tracts in California’s Sacramento-San Joaquin Delta (hereafter referred to as the “Delta”, Figure 1A, B). Cultivation of the Delta’s peat soils began in the late 1800s, as Delta marshes were drained and leveed to utilize the nutrient-rich soils for agriculture Subsidence occurs on these islands because drainage of the marsh soil leads to changes in the decomposition pathway. Under the drained conditions required for agricultural use, decomposition increases because oxygen is introduced to the matrix and the organic matter is converted to carbon dioxide and released as gas, leaving behind an altered soil matrix lower in organic content (oxidized peat). These same factors remain the primary cause of subsidence today (Deverel and Rojstaczer 1996)

Results

Discussion

Conclusion