Wetland Assimilation



Wetland assimilation is the discharge of an effluent (typically secondarily-treated municipal effluent) into natural wetlands.  Wetland ecosystems possess the unique capacity to safely and effectively remove nutrients, sediments, and other pollutants (e.g., pharmaceutical and personal care products, estrogenic compounds) from water sources via plant uptake, microbial processes, adsorption, sedimentation, and other processes.  In coastal Louisiana, where natural hydrology has been altered extensively due to oil and gas mining, flood control levees on the Mississippi River and other areas, and agricultural impoundment, trees and herbaceous vegetation thrives on treated effluent components (including nitrogen, phosphorus, sediments, and fresh water ) when properly introduced.  The result is a cost effective, environmentally sustainable approach for municipalities to meet their Louisiana Pollutant Discharge Elimination System (LPDES) permit limits.



Processes Involved

In the State of Louisiana, the Louisiana Department of Environmental Quality (LDEQ), in consultation with the United States Environmental Protection Agency (USEPA), regulates wastewater treatment and the discharge of treated effluents.  Over the past 15 years, scientists at Comite Resources, Inc., State regulatory personnel, and dischargers have worked closely to develop an approach where wetland assimilation systems meet water quality goals while protecting and restoring wetlands (Day et al. 2004). 

The process of establishing a wetland assimilation project begins with identification of a suitable candidate wetland.  All wetland ecosystems are not created equal and some are clearly unsuited for wetland assimilation.  The LDEQ has recognized several wetland types that are not appropriate for assimilation, including seasonally flooded pine flatlands with carnivorous plants and areas heavily used for recreation and oyster production.  There are a number of factors taken into consideration for site selection including location, size, hydrology, ecological condition and competing uses.  After a candidate wetland is selected, a preliminary feasibility study is conducted to determine if the discharge of treated effluent into the candidate wetland is possible (see figure below).

Steps in establishing an

assimilation wetland

Feasibility Study

The feasibility study usually lasts two to four months, depending upon the size and complexity of the potential assimilation wetland.  During the feasibility study, wetland characteristics (hydrology, soils, vegetation, fauna) are described, along with assessment of surrounding landscape uses, expected nutrient loading rates, and occurrence of protected flora and fauna, or archaeological and/or historical sites.  A preliminary conceptual design of the treated effluent distribution system is also included.  If the feasibility study finds the candidate wetland suitable for assimilation, a year-long ecological baseline study (EBS) is conducted. 

Ecological Baseline Study

The purpose of the EBS is to describe in greater detail the baseline ecological conditions of the candidate site, including hydrology, soil and water chemistry, and vegetative species composition.  In addition, a preliminary engineering design and cost analysis are conducted.  The EBS usually lasts one year.  The EBS then forms part of the permit application, which is the fourth step in the process.  However, the LPDES permit application can be submitted before the EBS is complete.


An LPDES permit is required under the authority of the Federal Clean Water Act and the Louisiana Environmental Quality Act, and this permit is issued by LDEQ.  These two acts require criteria (as set forth in the permit) to protect the beneficial uses (e.g., fish and wildlife propagation) and contain an anti-degradation policy that limits lowering of water quality.  The LPDES permit designates effluent limits for discharge to the wetland and outlines monitoring requirements.  Generally, effluent limits are considerably less than for direct discharge to a water body because of the ability of the wetland to process and assimilate nutrients and organic matter in the effluent (Day et al. 2004).  The permit requires disinfection so that pathogens are not discharged to wetlands and toxic materials must be below state and federal limits.  Other permits that may be required include the Louisiana Department of Natural Resources Coastal Use Permit, US Army Corps of Engineers Section 404 permit, a levee board permit, and an LDEQ Water Quality Certification.


After the permit is issued, infrastructure is built for the project (e.g., effluent distribution pipeline, monitoring boardwalks), discharge begins, and annual monitoring is started. Contamination by human pathogens is an important issue that must be considered in wetland assimilation since they can be transferred to other animal species as well as to humans.  For this reason municipal effluent is disinfected prior to release to receiving waters, or in this case, wetlands.  Nevertheless, numerous studies have shown that pathogens are rapidly degraded in wetlands, much more so than in receiving water bodies (i.e. lakes or streams).  Proper disinfection is a particular concern for all municipal effluent treatment plants, and dischargers have to regularly monitor the effectiveness of disinfection systems.           


Monitoring of vegetation, hydrology, water quality, and soils is a vital component of any wetland assimilation project.  Requirements for this monitoring are outlined in the LPDES permit for discharge of treated effluent into a wetland.  Vegetation data provide information on the health and vigor of the plant community, and whether vegetative species composition or dominance is being altered due to effluent addition.  Water gauge data provides information about hydrology and changes in the depth and duration of inundation.  Metals and nutrient data of soils and vegetation determine if there is a build up of these materials that could become problematic.  Surface water data provides information of the efficiency of the system in removing nutrients from the water column.  Data are collected in the assimilation wetland and in an ecologically similar reference wetland that is not impacted by the treated effluent.  By comparing data in the assimilation and reference wetlands, scientists can determine if the assimilation wetland is being impacted by effluent addition. 

According to the LPDES discharge permit, if wetland monitoring indicates that there is (A) more than a 20% decrease in naturally occurring litterfall or stem growth; or (B) significant decrease in the dominance index or stem density of baldcypress, then the permittee shall conduct such studies and tests as to determine if the impact to the wetland was caused by the effluent.  Thus, monitoring provides a mechanism for evaluating the impacts of treated effluent on an assimilation wetland.