Jason N. Day

WETLAND SURFACE ELEVATION, VERTICAL ACCRETION, AND SUBSIDENCE AT THREE LOUISIANA ESTUARIES RECEIVING DIVERTED MISSISSIPPI RIVER WATER

Wetland surface elevation and vertical accretion were measured from 1996 to 1999-–2000 using a sediment elevation table (SET) and feldspar marker horizons in nine paired wetlands receiving Mississippi River water from the Caernarvon, West Pointe a la Hache (WPH), and Violet river diversions. The Caernarvon study sites had wetland surface elevation change rates ranging from 0.16±0.31 to 0.42±0.21 cm y−1. Vertical accretion ranged from 0.75±0.04 to 1.57±0.05 cm y−1, and shallow subsidence ranged from 0.59 to 1.21 cm y−1. Wetland surface elevation at the WPH study sites initially increased 2.3 to 3.3 cm during the first seven months of the study and then steadily decreased over the following year. The overall rate of elevation change ranged from 0.27±0.09 to 0.70±0.11 cm y−1. Vertical accretion and shallow subsidence ranged from 1.24±0.08 to 1.84±0.07 cm y−1 and 0.54 to 1.27 cm y−1, respectively. The Violet sites lost elevation and had the highest subsidence rates in this study, most likely due to a combination of hydrologic alteration and low diversion discharge. Wetland elevation decreased throughout the study, with rates ranging from −1.10±0.24 to −2.34±0.41 cm y−1. Vertical accretion and shallow subsidence rates at the Violet-Near and Far sites were 0.44±0.10 and 0.44±0.11 cm y−1 and 2.78 to 1.54 cm y−1, respectively. The Violet-Mid site wetland was burned in Winter 1999, leading to more than 4.0 cm decrease in material measured over the marker horizon and contributing to the lowest accretion rate measured in this study of 0.34±0.05 cm y−1. Analysis of regional relative sea-level rise (RSLR) indicates that all Caernarvon sites and the WPH-Near and Mid sites are keeping pace with RSLR. This study indicates that the use of river diversions can be an effective coastal restoration tool, with efficiency related to the proximity to riverine source and degree of hydrologic alteration, quantity of river water released, and land uses of the receiving wetland basin. Landscape modifications such as spoil banks associated with oil and gas access canals negate the benefits of river water introduction by limiting wetland-water interaction and should be removed in conjunction with river diversion implementation for effective wetland restoration.

Potential nitrate removal from a river diversion into a Mississippi delta forested wetland

Abstract The objectives of this study were: (1) to carry out a baseline study of water quality parameters in the Maurepas forested wetland in Louisiana, USA; and (2) to estimate potential nitrate uptake of a proposed Mississippi River diversion into the wetland. Water sampling trips were carried out monthly from April to October 2000. Average water quality parameter concentrations and ranges were: nitrate 0.008 mg-N l−1 (non-detectable (n.d)-0.143 mg-N l−1); ammonium 0.007 mg-N l−1 (n.d-0.048 mg-N l−1); total nitrogen 0.577 mg-N l−1 (0.193–1.285 mg-N l−1); phosphate 0.034 mg-P l−1 (n.d-0.369 mg-P l−1); total phosphorus 0.055 mg-P l−1 (0.022–0.424 mg-P l−1); total suspended sediment 16 mg l−1 (4–101 mg l−1), salinity 3‰ (0–12‰), and chlorophyll a 11 μg l−1 (1–31 μg l−1). A UNET hydrodynamic model was constructed to predict hydrologic patterns as diverted water flowed through the Maurepas swamp. The study area was divided into 53 storage cells based on topographical features that mostly consisted of natural bayous and degraded artificial levees. Nitrate loading was high in the initial cells and removal efficiencies were on the order of 40–70%. Loading in subsequent cells was much lower and simulated nitrate retention was greater than 90%. Since most nutrients will be retained in the swamp, the proposed diversion of Mississippi River water should not cause adverse water quality conditions or extreme or persistent algal blooms in the Lake Maurepas.

Short-term sedimentation dynamics in the Rhône River Delta. France: The importance of riverine pulsing

Short-term sedimentation patterns were evaluated from August 1992 to May 1993 in different wetland habitats characteristic of the Rhône Delta, including impounded and seasonally-dry saline Arthrocnemum marshes, brackish Juncus, Phragmites, and Scirpus riverine wetlands directly connected to the Rhône River, and Arthrocnemum-dominated marine marshes influenced by the Mediterranean. Short-term sedimentation was measured as sediment accumulation on paper filters which had been placed on the soil surface for several weeks. Total sedimentation and material lost on ignition was significantly related to individual sampling periods, reflecting the importance of short-term processes. High sedimentation at riverine sites (up to 22 g m−2 d−1) was related to a combination of river stage and wind events. Marine and impounded wetlands of the Rhône Delta experienced low sedimentation throughout the period of study. Sedimentation rates averaged over the study period were 0.8 g m−2 d−1, 1.8 g m−2 d−1, and 5.4 g m−2 d−1 for marine, impounded, and riverine sites, respectively. Percent material lost on ignition was low in all habitats (average 15%) and followed a seasonal trend with a minimum in late fall and winter (1%). Soil percent organic matter was also low in the top several centimeters (13%), suggesting that inorganic sedimentation is very important for accretion on these wetland surfaces. Coastal flooding was not a significant mechanism for sedimentation in the marine sites during the period of study. Sedimentation is an important factor in elevation change, and this study shows that impounded habitats, the most common “natural environment“ left in the delta, may become vulnerable to sea-level rise in the future if management practices continue to isolate these wetlands from riverine sources of sediment.

Impacts of Secondarily Treated Municipal Effluent on a Freshwater Forested Wetland After 60 Years of Discharge

Secondarily treated municipal effluent from Breaux Bridge, Louisiana has been discharged into the Cypriere Perdue forested wetland since the early 1950s. Approximately one million gallons per day (3,785 m−3 day−1) are discharged into the 1470 ha wetland, with average total nitrogen and phosphorus loading rates of 1.15 g N m−2 yr−1 and 0.31 g P m−2 yr−1, respectively. Vegetation and water quality of this wetland, along with a reference wetland, were monitored. Study sites were dominated by bald cypress and water tupelo, and species composition did not change significantly during the time of monitoring. Mean litterfall was higher near the effluent discharge point compared to sites located further away or the reference site. Mean stem growth was lower at the site furthest from the discharge point compared to the other sites. Nutrient concentrations measured at the site where water exits the assimilation area and at the reference site were not significantly different. Removal efficiencies for total nitrogen and phosphorus are typical of other forested wetlands receiving treated effluent in Louisiana, ranging between 65 and 90%. These results demonstrate that this wetland assimilates nutrients to background concentrations even after 60 years of operation, stimulating productivity, and causing no measurable impacts to the wetland or to the river into which the water eventually flows.

Using Natural Wetlands for Municipal Effluent Assimilation: A Half-Century of Experience for the Mississippi River Delta and Surrounding Environs

An assimilation wetland is a natural (non-constructed) wetland into which secondarily-treated, disinfected, non-toxic municipal effluent is discharged. In the Mississippi River Delta, the wetland is typically either a freshwater forested wetland (e.g., baldcypress-water tupelo) or a freshwater emergent wetland. These wetlands have been hydrology altered, some extensively, with freshwater input reduced from historical norms. Discharge of freshwater effluent with nutrients and suspended sediments into an assimilation wetland increases vegetation productivity and accretion and combats subsidence. Effluent discharge rate into an assimilation wetland depends on wetland size and effluent nutrient concentrations. Design and construction of an assimilation wetland requires a Louisiana Department of Natural Resources (LDNR) Coastal Use Permit (CUP), a Louisiana Department of Environmental Quality (LDEQ) Louisiana Pollutant Discharge Elimination System (LPDES) permit, a US Army Corps of Engineers (USACE) 404 permit, and an LDEQ Water Quality Certification, along with potential levee board permit applications. Both a feasibility study and an ecological baseline study are conducted before discharge of treated effluent begins. Assimilation wetlands are designed with a minimum of four monitoring sites; three located along a transect from the discharge to the area where surface water leaves the wetland, and the fourth, a reference area, located in an ecologically similar wetland nearby. As part of the LDEQ LPDES permit, study sites within an assimilation wetland are monitored continually for the life of the project, including vegetation productivity and species composition, sediment accretion, hydrology, and surface water nutrient and metals concentrations. There are ten active assimilation wetlands in coastal Louisiana and another four with permit applications pending. Results of annual monitoring show nutrient concentrations of surface waters decrease with distance, reaching background levels before water leaves the wetland. While nutrient concentrations decrease, vegetative productivity is enhanced. In degraded forested wetlands being used as assimilation wetlands, baldcypress and water tupelo seedlings are often planted, which thrive in the nutrient rich environment. However, nutria are attracted to vegetation with increased nutrient concentrations, and herbivory severely damaged one emergent wetland receiving municipal effluent, killing both herbaceous vegetation and unprotected tree seedlings. After culling of nutria, the wetland recovered. This introduced species must be monitored and controlled in any assimilation wetland. Here we review the history of assimilation wetlands in the Mississippi River Delta to show how advances in scientific understanding, growing regulatory sophistication, and controversy have shaped this program.