Low Intensity Chemical Dosing (LICD)

Science Center Objects

Rivers, wetlands, and agricultural operations supply natural organic material to the Sacramento-San Joaquin Delta (Delta) and the San Francisco Estuary. This natural organic matter provides many ecosystem benefits, but it also adversely affects drinking water. During drinking water treatment, chlorine added for purposes of pathogen control reacts with dissolved organic carbon (DOC) in the water to form carcinogenic and mutagenic disinfection by-products (DBPs), which are regulated in tap water by the U.S. Environmental Protection Agency.

On site coagulation with metal based salts, or low-intensity chemical-dosing (LICD), is anticipated to remove considerable amounts of DOC and DBP precursors from drainage water, thereby significantly reducing inputs of these constituents to the Delta. Coagulation may also remove mercury from solution. Accumulation of the flocculated material in constructed wetlands, along with sequestration of wetland plant material, is expected to increase land-surface elevations. The project hypothesis is the combination of coagulation and biotic wetland processes can improve water quality and reverse subsidence beyond that achievable by either technology alone.

Picture of coagulation treatment cells

Coagulation treatment cells. (Public domain.)

Levee Failure   |   Study Design   |   Water Quality    |   Accretion   |   Ecosystem   |   Feasibility   

 

Map of the Sacramento-San Joaquin Delta

Map of the Sacramento-San Joaquin Delta.

Improving Delta Water Quality While Lowering Risks of Bay-Delta Levee Failure

Historically, the Sacramento—San Joaquin Delta (Delta) in California was a vast inland freshwater wetland where organic soils over 50 feet deep formed over several millennia. Beginning in the late 1800’s, levees were constructed and pumps were used to drain the area for agricultural use. Draining these lands has led to land-surface subsidence to more than 20 feet below sea level in some areas, primarily due to oxidative loss of the organic rich soils. Because the Delta supplies drinking water for over 23 million Californians, protecting both water supply and water quality in this region is of great importance.

Dissolved Organic Matter and Water Quality

Rivers, wetlands, and agricultural operations supply natural organic material to the Sacramento-San Joaquin Delta (Delta) and the San Francisco Estuary. This natural organic matter provides many ecosystem benefits, but it also adversely affects drinking water. This occurs because during drinking water treatment, chlorine added for purposes of pathogen control dissolved organic carbon (DOC) in the water to form carcinogenic and mutagenic disinfection by-products (DBPs). Concentrations of these compounds in tap water are regulated by the Environmental Protection Agency.

Because much of the land in the Delta is below sea level, drainage water must be continuously pumped into adjacent Delta channels to prevent these areas from flooding. This drainage water typically contains high concentrations of DOC and DBP precursors because they have passed over and through high organic matter peat soils (Fujii et al. 1998, Fleck et al. 2007). Drainage water from Delta peat islands has been shown to represent a significant source of these constituents of concern to drinking water diverted from the Delta (Kraus et al. 2009). Management actions that reduce the export of these constituents from subsided islands will improve water quality in the Delta.

Land Subsidence and Levee Stability

Land subsidence is a dropping of the land surface. While land-surface loss can be due to dewatering, groundwater pumping, or soil compaction, in the Delta the dominant cause is loss of the organic-rich peat soils due to microbial degradation. Put simply, microbial activity — which is enhanced under drained (oxygen rich) conditions compared to flooded (oxygen poor) conditions — converts the organic rich soil to carbon dioxide gas (CO2).

As land surface elevations decrease, costs for levee maintenance and repair increase, as do the risks of catastrophic levee failure. Currently 98 percent of the Delta is below sea-level (Knowles 2010). In addition to immediate loss of life and property associated with levee failure, saltwater intrusion into this freshwater system could result and halt water exports for an extended period of time (Mount and Twiss 2005). Management actions that reduce, or better yet reverse, subsidence in the Delta would reduce these costs and risks.

The Solution

The primary goals of this project are to assess the feasibility of the following: 

  • Treating Delta island drainage water with low concentrations of coagulants to remove dissolved organic carbon, disinfection byproduct precursors, and other constituents of concern (e.g. mercury) prior to release into Delta Channels.
  • Accumulating the resulting metal-organic matter flocculate (the material that forms coagulation is a metal-organic matter complex that precipitates out of solutions) in constructed wetlands managed to reverse subsidence. 

This low-intensity chemical-dosing (LICD) method is anticipated to remove considerable amounts of DOC and DBP precursors from island drainage water, thereby significantly reducing inputs of these constituents to the Delta. Accumulation of the flocculated material in wetlands, along with sequestration of wetland plant material, is expected to increase land-surface elevations. The project hypothesis is that the combination of coagulation and biotic wetland processes can improve water quality and reverse subsidence beyond that achievable by either technology alone.

A graphic of a wetland-cell to improve water quality

An example of a wetland-cell design (not to scale). Nine experimental-wetlands cells were constructed on Twitchell Island, a subsided island in the Sacramento–San Joaquin Delta, to test the effectiveness of using metal-based coagulants to improve water quality in the Bay-Delta.

Back to the top >>

 

Studying the Coagulation-Wetland System's Effects in the Field: Water Quality, Subsidence Reversal, Carbon Fate and Transport, and Ecosystem Health

A schematic of experimental-wetland cells

Schematic of the nine experimental-wetlands cells constructed on Twitchell Island. Each wetland cell in this field experiment received either untreated island drainage water (control), drainage water treated with iron sulfate (iron), or drainage water treated with polyaluminum chloride (aluminum). (Public domain.)

This project combines laboratory and field studies to assess the feasibility and ecosystem effects of LICD. Specific study objectives will includes assessments of water quality, accretion, and ecosytem dynamics. Laboratory studies will assess the efficacy of coagulant type and dosing rates for removing DOC, DBP precursors, and other constituents of concern (e.g. mercury) from island drainage water, as well as assess characteristics and stability of the flocculated material.

Construction of a replicated field experiment located on Twitchell Island will allow us to determine the effects of a coagulation treatment-wetland system on water quality. The experimental design includes three coagulation treatments—an iron-based coagulant (iron sulfate), an aluminum-based coagulant (polyaluminum chloride), and a control (no coagulant addition). There are three replicates of each treatment for a total of nine wetland cells. This system is expected to run from Fall 2011 to Fall 2013. The effects of the treatment-wetland systems will be assessed by monitoring water quality, sediment accretion, plant production, and aquatic organism health.

The primary goals of this project are to assess the feasibility of the following:

  • Treating Delta island drainage water with low concentrations of coagulants to remove dissolved organic carbon, disinfection byproduct precursors, and other constituents of concern (e.g. mercury) prior to release into Delta Channels.
  • Accumulating the resulting metal-organic matter flocculate (the material that forms following coagulation, is a metal-organic matter complex that precipitates out of solution) in constructed wetlands managed to reverse subsidence.

Experimental Design

The demonstration coagulation-wetland field study is composed of nine cells that include three coagulation treatments, each replicated three times. The three coagulation treatments include the following:

  1. Iron-based coagulant (ferric sulfate, FS).
  2. Aluminum-based coagulant (polyalumnium chloride, PAC).
  3. Control, which receives uncoagulated drainage water (no coagulant).

Sampling Approach

Field experiments will focus on organic carbon fate and transport as well as assess potential ecosystem effects. All compartments (water, sediment, vegetation) will be sampled to achieve these objectives.

Water samples will be collected regularly prior to coagulation, following coagulation, and after passage through the wetlands (wetland outflows). In addition, water samples will be collected across the cells to examine changes in water quality during passage through the wetlands. Water samples will be analyzed for dissolved and particulate organic matter, disinfection by-product formation, nutrients, and metals.

Sediment samples will be collected to determine rates of accretion as well as the composition and quality of the accreted material. Sediment carbon, nutrient, and metal concentrations will be determined along with bulk density.

Plant samples will be collected to assess whether the coagulation treatments influence plant primary productivity.

Picture of coagulation cells for three different dates

Coagulation cells over time.

Back to the top >>

 

The Effects of the Coagulation-Wetland System on Water Quality

A picture of water quality samples

Water samples collected from the inflows (in) and outflows (out) of the coagulation treatment wetland cells. Al = source water treated with polyaluminum chloride. Fe = water treated with iron sulfate. Control = untreated.

The primary objective of this part of the study is to determine if the coagulation-wetland systems reduce DOC and DBP precursor loads from island drainage water. This will be accomplished both through regular water-quality monitoring of the inlet and outlet of each cell, as well as through synoptic studies focused on the production, loss, and transformation of constituents during transport through the wetland cells. Surface-water, pore-water, vegetation and sediment samples will be collected and analyzed to address these objectives.

Data will be used to develop a mass-balance model of DOC, DBP precursors, nutrients, and dosed chemicals (aluminum, Al, or iron, Fe) in the system. In addition, more detailed characterization of the DOC pool will help us determine the sources and processing of material within the wetland to assess whether plants, soils or algae are contributing DOC and DBP precursors.

 

 

Water Quality and Loads Objectives for the LICD Project
Primary Objective Sub-Objectives   Approach
Determine if the coagulation-wetland systems reduce dissolved organic carbon (DOC), disinfection by-product (DBP) precursor, mercury, and other constituent concentrations and loads from island drainage waters. WQ & Loads-1 (Ecosystem-6) Quantify the DOC, DBP precursor, total suspended solids (TSS), and mercury concentration and load reduction achieved by coagulation alone, and compare results between Fe and Al based coagulants. Field/Lab
  WQ & Loads-2 (Ecosystem-6) Quantify and compare changes in DOC, DBP precursor, mercury, and nutrient concentrations and loads due to passage of untreated and treated island drainage water through the coagulation-wetland systems. Field
  WQ & Loads-3 Quantify and compare changes in TSS and POC concentrations and loads due to passage of island drainage water throutgh the coagulation-wetlands systems. Field
  WQ & Loads-4 Quantify the addition of DOC from the wetland systems and, if possible, discriminate between contributions from vascular plants, algal production, and DOC release from peat soils. Assess whether subsurface DOC exports originating from peat soils are reduced in coagulant-amended wetland soils. Field/Lab

 

Back to the top >>

 

A map depicting subsidence in the Sacramento-Joaquin Delta

Subsidence in the Sacramento-San Joaquin Delta. (Public domain.)

Can Coagulation Help Slow Land Subsidence in the Delta? 

In order to halt or reverse subsidence, accumulation of material has to equal or exceed its loss. In the case of organic matter, production of organic matter by photosynthesis (i.e. plant growth) must exceed its loss through degradation by microorganisms. Wetlands that support emergent vegetation produce high rates of plant biomass (net primary production, NPP), whereas flooded conditions create oxygen-depleted (anaerobic) conditions which slow decomposition rates—the net effect of this is accumulation of organic material.

Prior research conducted by the USGS on Twitchell Island has shown that constructed wetlands can increase land-surface elevations up to 3.5 inches per year (Miller et al. 2008). The LICD project is investigating whether a combination of wetland plant production and the addition of the metal-organic matter flocculate resulting from coagulation can exceed rates of land surface accumulation achieved by wetland plants alone.

Accretion Objectives for the LICD Project
Primary Objective Sub-Objectives Approach
Determine whether wetland accretion rates and long-term sediment stability will be augmented by the coagulation-wetland system compared to a wetland alone. Accretion-1 Quantify and compare sediment accretion rates for the three treatments (control vs iron vs aluminium). Field
Accretion-2 (Ecosystem-4) Determine if there are any effects of coagulant addition on plant growth. Field
Accretion-3 Assess effects of coagulant addition on sediment composition and stability (e.g. carbon retention, lability, nutrients, iron, aluminium). Field/Lab

 

Back to the top >

 

Assessing Habitat Safety for Delta Wildlife

Research under the "Ecosystem Objective" for this project will examine wheather the addition of coagulated drainage water affrects ecosytem health. the addition of the iron or aluminum-based coagulants could directly or indirectlt affect several things, including pH, redox conditions, nutrient availability and cycling, plant growth, and aquatic toxicity. The primary goal is to improve water quality by removal of constituents of concern (DOC, DBP precursors, mercury) during passage of island drainage water through the coagulation-wetland system. Conditions within the wetlands themselves, however, should provide a safe habitat for aquatic and other wildlife and alos lead to the beneficial accumulation of soil material.

By determining whether there are differences in plant growth and sediment accretion rates between the different wetland treatments (iron coagulant vs, alluminum coagulant vs. verses no coagulant) we can assess whether any measured differences in the water quality are affecting rates of primary production and decomposition. Effects on aquatic organisms will be determined through EPA toxicity tests.

Mercury

Concentrations of mercury (Hg) particularly methyl mercury (MeHg), are also of great concern in the Delta. Sources of HG include historic inputs from the Gold Rush era, drainage from abandoned mines, and ongoing air deposition from power plants and industry. Recent dissolved in subsided-island drainage water (Hennebery et al., 2011: Hennebery et al., 2016). Field studies showed similar reductions along with lower methlmercury bioaccumulation in fish (Ackerman et al., 2015).

Ecosystem Effects Objectives for the LICD Project
Primary Objective Sub-Objectives Approach
Assess whether there are toxicity issues relating to surface-water export from the coagulation-wetland system as well as within the wetlands themselves. Ecosystem-1 Determine whether wetlands receiving coagulants export reactive particulates or high metal concentrations that could have adverse downstream impacts. Field
Ecosystem-2 Determine whether water quality within wetlands is altered by coagulant addition (e.g., temperature, pH, nutreints, metals). Field
Ecosystem-3 (Accretion-2) Determine if there are any effects of coagulant addition on plant growth. Field
Ecosystem-4 (Accretion-3) Assess effects of coagulant addition on sediment composition and stablility. Field/Lab
Ecosystem-5 Investigate the effects of (a) coagulant addition alone and (b) the coagulation-wetland system on total and methyl mercury concentrations and loads. Field/Lab
Ecosystem-6 (Loads 1 & 2) Determine effects of the coagulation-wetland system on methylmercury bioaccumulation in fish. Field
Ecosystem-7 Assess effects of coagulant additions on gas fluxes, including carbon monoxide, methane, and nitrogen oxides. Field

 

Back to the top >>

 

Economic Feasibility of a Coagulation - Wetland System

The costs associated with constructing and maintaining a coagulation-wetland system will be calculated in order to assess the feasibility of implementing this system to improve water quality and reverse subsidence in the Delta.

Economic Feasibility Objectives for the LICD Project
Primary Objective Sub-Objectives Approach
Conduct a cost/benefit analysis of implementing LICD systems to treat island drainage water. Determine costs associated with constructing and maintaining a coagulation system. Costs and benefits will be determined for (a) the coagulation system alone and (b) coagulation plus a wetland system. Field