MONITORING AND RESEARCH FOR MITIGATION OF SUBSIDENCE ON DELTA ISLANDS

The CMARP workteam for subsidence mitigation developed the following recommendations for data collection, monitoring and research for subsidence mitigation in the Delta. The members of the workteam are Steve Deverel (HydroFocus, Inc.), Roger Fujii (USGS) and Lauren Hastings (USGS). The workteam first developed conceptual models and identified data gaps for five subject areas related to subsidence mitigation. The five subject areas are 1) causes and rates of subsidence, 2) peat thickness, 3) priority areas for subsidence, 4) effects of future subsidence on land use and water quality and 5) land- and water-management practices for subsidence mitigation. Based on the conceptual models for each topical area and the data gaps, directions for monitoring, data collection and research are described for each subject area.

Conceptual Models and Data Gaps

Causes and rates of subsidence

The available data indicate that the present-day subsidence of peat soils is primarily caused by oxidation of organic carbon. Subsidence has slowed from the historic, time-averaged subsidence rates that ranged from 0.5 to over 4 inches per year from the early 1900's to the 1970's and 1980's. Oxidation of organic carbon appears to follow Michaelis-Menton kinetics, such that the rate of oxidation and subsidence is proportional to the amount of carbon in the soil. As the soil organic carbon content decreases, the oxidation and subsidence rates also decrease. Consolidation also causes subsidence as the result of continued lowering of drainage ditch elevations to compensate for subsidence. This reduces the buoyant force of water on the peat soils and causes a loss of volume of the peat soil.

Subsidence rates in the Delta were last determined on Bacon and Mildred Islands and Lower Jones Tract in 1981. Subsidence rates on these islands ranged from 1.2 to 1.6 inch per year in 1980. The historic, time-averaged rates from 1924 through 1981 ranged from 2 to 3 inches per year. Data collection on Sherman and Jersey Islands in 1987 demonstrated that time-averaged subsidence rates ranged from 0.5 to 1.2 inches per year from 1910 to 1987 and ranged from less than 0.3 to 0.7 inch per year from 1952 to 1987. Subsidence rates have generally been lower in the western Delta relative to the central Delta.

Accurate estimates of present-day subsidence rates and prediction of future subsidence rates are important for meeting CALFED Program Goals and Objectives. For example, implementation of objective of the Delta Island Subsidence Control Plan "to reduce the risk to levee stability from subsidence" requires current data for subsidence rates. To estimate the effects of future subsidence on levee stability in peat soils adjacent to levees, present-day subsidence rates need to be accurately determined and future subsidence rates need to be predicted with reasonable accuracy. Reasonable estimates of present-day and future rates of subsidence are also essential for prioritizing areas for subsidence mitigation.

Data Gaps

  • Present-day subsidence rates throughout the Delta for peat soils need to be quantified
  • Improved quantification of the processes causing subsidence, specifically consolidation and microbial oxidation. Additional data collection and analysis are required to develop the parameters for the equations describing the rates of oxidation to fully understand the oxidation process and predict future subsidence rates.
  • The ongoing volume loss in peat soils due to consolidation and shrinkage needs to be quantified.

Peat Thickness

The current values for peat thickness were estimated by extrapolating data for the basal elevation of the peat deposits from borehole data mapped by Atwater (1982) throughout the Delta. Generally, peat thickness is greatest in the western Delta and decreases towards the eastern Delta. The basal peat elevations were subtracted from land-surface elevations to estimate peat thickness. The data are based on land-surface elevations determined in 1974 and 1975 are in error due to subsidence that has occurred during the last 24 years. There is also error due to spatially incomplete data for the basal elevation of the peat deposits. The peat thickness is a key factor influencing the delineation of priority areas for subsidence mitigation because of its relation to how much the land-surface elevation will decrease upon disappearance of the peat soil.

Data gap

  • Data for peat thickness for peat soils in the Delta

Delineation of Priority Areas for Subsidence Mitigation

Priority areas for subsidence mitigation were heretofore based the distribution of time-averaged subsidence rates from the early 1900's to the mid-1970's. Because of the error in time-averaged subsidence rates and peat thickness, the delineation of priority areas can be in error. The largest errors are in the western Delta and other locations where the time-averaged subsidence rates from the early 1900’s to the 1970’s and 1980’s are 1.5 inch per year or less. Also, time-averaged subsidence rates have decreased over time and prioritization for subsidence mitigation should be based on present-day rates of subsidence.

Data gap

  • Delineation of priority areas for future data collection and subsidence mitigation based on present-day subsidence rates and peat thickness.

Future Effects of Subsidence on Land Use and Water Quality

As islands continue to subside, depths of drainage ditches on islands will need to be deeper to prevent flooding and to maintain aerobic conditions in the crop root zone for continued agricultural production and other non-flooded land uses. The hydraulic gradient from the channel water surface to groundwater in the center of the subsiding island will increase, thereby increasing seepage through the levee and onto the island. Increased seepage onto islands in the Delta may affect levee stability and increase the potential for island flooding. Flooding causes property damage and can disrupt power transmission, water conveyance and exploration and transport of natural gas. The costs of reclaiming an island after flooding range into the millions of dollars and several Delta islands have not been reclaimed after flooding. The effects of future subsidence on levee seepage and deformation have not been quantified.

Seepage onto Delta islands can directly affect land use. Although the data are limited, seepage volumes appear to be large. In 1995, drainage volumes in the central and western Delta ranged from 2 to 4 acre-feet per acre. Increased subsidence will increase the volume of seepage onto Delta islands and pumping costs. Continued subsidence may limit the ability of the current drainage infrastructure to remove seepage. This may result in the need for infrastructure improvements such as additional pumping stations and drainage ditches. If improvements are not made, there may be areas that will become permanently flooded because the drainage systems will be unable to effectively drain islands. Seepage onto Delta islands has not been quantified.

Increased seepage will also result in increased volumes of drainage water and increased loads of dissolved organic carbon (DOC) and disinfection byproduct precursors (DBPP) (e.g. trihalomethane precursors or THMPs) that are returned to channel waters, degrading the quality of water subsequently exported for drinking water. (The concentrations of trihalomethanes (THMs) are currently regulated in drinking water supplies. However, disinfection byproducts in addition to THMs such as haloacetic acids are scheduled to be regulated and should also be considered for evaluation.)

Another important consequence of levee failure and island flooding is seawater intrusion. Preliminary estimates by the USGS (Larry Smith, personal communication, 1998) indicate that flooding of two to three Delta islands of median size and depth will double the tidal prism (the amount of seawater flowing past Chipps Island between slack low water and slack high water) during normal to low river flow conditions. This volume of water that moves onto an island during flooding will increase as islands continue to subside. Increased seawater intrusion due to levee failures will have detrimental effects on ecosystem habitats and drinking water supply.

Continued subsidence will also limit the possibilities for future ecosystem restoration in the Delta. There is currently no ecological habitat connection between the wetland habitat in Suisun Marsh west of the Delta and the riparian habitat east of the Delta. Mitigation of subsidence over the long term should take into account the need for an ecological connection between these two ecosystems adjacent to theDelta. Continued subsidence will increase the time and difficulty associated with restoring land-surfaces and the development of tidal wetlands.

Data gaps

  • Hydrologic inputs and outputs for Delta islands, including seepage, drainflows, irrigation diversions and crop consumptive use.
  • Quantification of the effects present-day and future seepage on levee stability.
  • Quantification of the effects of future subsidence on levee deformation.
  • Economic analysis of effects of continued subsidence on agricultural production.
  • Drainage volumes from Delta islands coupled with DOC and THMFP concentrations need to be quantified and monitored to estimate DOC and DBPP loads (concentration times volume) from the Delta islands.
  • Quantification of the effects of continued subsidence on water quality effects caused by flooding of Delta islands.

Land- and Water-Management Practices for Subsidence Mitigation

Options for mitigating subsidence in the Sacramento-San Joaquin Delta are based on two key concepts. First, present-day subsidence of the Delta peat soils primarily is the result of microbial oxidation of soil organic matter, and microbial oxidation of soil organic matter in Delta peat soils is dependent on soil moisture and temperature. Second, oxidation rates are directly proportional to soil temperature. Also, oxidation rates are generally highest when soil volumetric moisture contents are between 30% and 50% and they decrease above and below these levels. Microbial oxidation of peat soil decreases substantially under saturated and flooded conditions. Options for reducing and stopping subsidence in the Delta organic soils are based on reducing the rate of carbon loss due to microbial oxidation or soil organic carbon. Reversing the effects of subsidence may be achieved by accumulating biomass or depositing mineral or organic material to accrete the land surface. Possible land- and water-use options for reducing, stopping or reversing subsidence include, permanent shallow flooding, reverse flooding, deep flooding to create open-water habitat, saturated pasture, accretion of the land surface with imported biomass and mineral capping of peat soils.

The results of studies conducted by the US Geological Survey and Department of Water Resources (Deverel and others, 1998) demonstrated that permanent shallow flooding resulted in a net carbon accumulation and accretion of biomass. Other water-management strategies that were evaluated, seasonal flooding during the late fall and winter with and without irrigation during the spring and summer, resulted in a net carbon loss and are not viable mitigation strategies for stopping subsidence. Results from this study lead to development of a 15-acre demonstration project on Twitchell Island that began in 1997. The overall objective of this project is to demonstrate the feasibility of developing wetland environments in peat soils that will reverse the effects of subsidence and lead to accretion of the land surface.

Other water- and land-management strategies are being evaluated that may stop, or reverse, the effects of subsidence, include capping the organic soil with mineral material and reverse wetland flooding. Preliminary results by the USGS (Lauren Hastings, personal communication, 1998) indicate that capping the unsaturated peat soil with 2 feet of dredge sand reduces the carbon dioxide emissions by about 35%. Capping of partially saturated soil reduced carbon dioxide emissions by 23%. Capping saturated peat soil with dredge material would provide upland habitat in shallow flooded wetlands. Capping of the peat reduces the transport of oxygen into and carbon dioxide out of the soil causing the rate of carbon dioxide emission, a function of organic matter oxidation, to decrease. Reverse wetland flooding involves shallow flooding during the spring and summer and drainage during the fall and winter. This may reduce oxidation when it is usually the greatest and result in organic matter accumulation. The USGS is currently evaluating this as a subsidence mitigation strategy. Gaseous carbon emissions for open water habitat (flooding to 2 feet or greater) also are being evaluated on Twitchell Island. While open water habitat should stop subsidence, the effects of subsidence are not expected to be reversed under this management practice because of the limited biomass accumulation in deep-flooded conditions.

Data gaps

  • Effects of different vegetation and water-management practices on biomass accretion
  • Long term biomass and land-surface accretion rates.
  • Feasibility of large-scale application of biomass accretion.
  • Effectiveness of other land- and water-management practices that can be used to mitigate subsidence such as reverse flooding and wet pasture.
  • Feasibility of using dredge materials for reversing the effects of subsidence and reducing microbial oxidation of peat soils.
  • Effects of applying dredge material to peat soils.
  • Effectiveness of sediment transport onto Delta islands for reversing the effects of subsidence.
  • What uses can be made of areas where dredge material has been applied.
  • Water quality effects of biomass accretion for subsidence mitigation.

Data Collection, Monitoring and Research Elements

Causes and rates of subsidence

  1. Estimate the distribution of present-day subsidence rates by releveling locations where historic elevations have been measured such as Bacon and Mildred Islands and Lower Jones Tract. Determine the areal distribution of soil organic matter contents throughout the Delta and use the correlation with subsidence rates to estimate the areal distribution of subsidence rates.
  1. Develop a leveling network to periodically measure changes in land-surface elevations. This network should be tied to the recent GPS survey. The network locations could be prioritized based on the distribution of historic subsidence rates and peat thickness.
  1. Water-level measurements in observation wells screened in the peat should be made concomitantly with the land-surface elevation measurements. Available data show that land-surface elevations fluctuate with changing groundwater elevations.
  1. Collect the data and develop the tools for predicting future subsidence rates. Analysis of soil samples and field measured oxidation rates from different locations to quantify the factors causing variability in oxidation rates will lead to the development of parameters for predicting future rates of subsidence. Data for the composition of the soil organic matter, oxidation rates, soil temperature and moisture content need to be collected in several locations throughout the Delta.

Peat Thickness

  1. The following data need to be collected, updated and analyzed for an accurate mapping of the peat thickness distribution in the Delta.
  1. Gather and compile borehole data collected since 1980 (Atwater, 1982, compiled data collected up to 1980) in an attempt to fill data gaps.
  1. Use surface geophysical methods to estimate the current distribution of peat thickness and fill in areas where data are lacking. Ground penetrating radar has been used successfully in Florida to determine the thickness of peat deposits. Limited borehole data will be used to verify the results of geophysical data collection efforts.

Delineation of Priority Areas for Subsidence Mitigation and Future Data Collection Efforts

  1. Redefine priority areas for subsidence mitigation based updated data for present-day subsidence rates and peat thickness
  1. Utilize the current GIS at DWR to store and analyze data for subsidence rates and peat thickness. Refine priority areas accordingly.
  1. The redefinition of priority areas should begin in areas where there is currently substantial error in the delineation of priority areas as defined in the 1998 report "Subsidence Mitigation in the Sacramento-San Joaquin Delta".

Future Effects of Subsidence on Land Use and Water Quality

  1. The rates of seepage onto Delta Islands need to be quantified by collecting and analyzing hydrologic data. Water level data in observation wells and aquifer testing will provide essential data for hydraulic gradients and transmissive and storage properties of subsurface materials that will enable the use of quantative techniques such as groundwater flow modeling to calculate seepage. Future subsidence can be put into the model to calculate its effects on future seepage. Because the volume of water pumped from the islands is seepage and agricultural drainage, additional data need to be collected to quantify drainage. Specifically, the volumes of irrigation water being applied, crop consumptive use and volumes of drainage water need to be determined. These data with the modeling can be used to calculate increased pumping and drainage needs.
  1. To determine the land-use effects of future increased seepage as the result of subsidence, economic analyses is needed to determine the effects of future increased pumping on the future viability of agricultural production in the Delta. There may also be water-quality related impacts due to future limits on drainage discharges to the Delta channels that need to be quantified.
  1. Assess and monitor drainage volumes and concentrations of DOC and DBPP to quantify loads of DOC and DBPP in drainage from islands. Although considerable DOC and THM formation potential data exist and are still being collected for selected drains and pumps in the Delta, drainage volumes from islands coupled with DOC and THMFP concentrations are not being monitored. The approach for this element involves initial research for each island investigated to determine loads of DOC and THMP in island drainage (drainage-return flows) and to develop necessary relations between pumpage and power-consumption data and water quality parameters and THMP for future monitoring.
  1. Assess the water quality effects of continued subsidence related to island flooding and salt-water intrusion.

Land- and Water-Management Practices for Subsidence Mitigation