Robert R. Lane

Water Quality Analysis of a Freshwater Diversion at Caernarvon, Louisiana

Since 1991, Mississippi River water has been diverted at Caernarvon, Louisiana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km2 of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, total Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton Sound from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and decreased salinity concentrations in the estuary due to the diversion. Spring and summer water quality transects indicated rapid reduction of nitrite + nitrate and total suspended sediment concentration as diverted Mississippi River water entered the estuary, suggesting near complete assimilation of these constituents by the ecosystem. Loading rates of nitrite + nitrate (5.6–13.4 g m−2 yr−1), total nitrogen (8.9–23.4 g m−2 yr−1), and total phosphorus (0.9–2.0 g m−2 yr−1) were calculated along with removal efficiencies for these constituents (nitrite + nitrate 88–97%; total nitrogen 32–57%; total phosphorus 0–46%). The low impact of the diversion on water quality in the Breton Sound estuary, along with assimilation of TSS over a very short distance, suggests that more water may be introduced into the estuary without detrimental affects. This would be necessary if freshwater diversions are to be used to distribute nitrients and sediments into the lower reaches of the estuary, in an effort to compensate for relative sea-level rise, and reverse the current trend of rapid loss of wetlands in coastal Louisiana.

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.

The Impacts of Pulsed Reintroduction of River Water on a Mississippi Delta Coastal Basin

Abstract During the twentieth century about 25% of the wetlands of the Mississippi delta was lost, partially a result of isolation of the river from the delta. River diversions are being implemented to reintroduce river water to the delta plain. We synthesize here the results of extensive studies on a river diversion at Caernarvon, Louisiana, one of the largest diversions in the delta.

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.

Seasonal and spatial water quality changes in the outflow plume of the Atchafalaya River, Louisiana, USA

The objective of this study was to examine the interaction between the Atchafalaya River and the Atchafalaya Delta estuarine complex. Measurements of suspended sediments, inorganic nutrients (NO3−, NH4+, PO43−), chlorophylla (chla), and-salinity were taken monthly from December 1996 to January 1998. These data were compiled by season, and the Atchafalaya River plume data were also analyzed using the Generalized Additive Model technique. There were significant decreases in NO3− concentrations during summer, fall, and winter as river water passed through the estuary, that were attributable to chemical and biological processes rather than dilution with ambient water. In some regions there were higher chla concentrations during summer and fall compared to winter and spring, when river discharge and the introduction of inorganic nutrients were highest, suggesting biological processes were active during this study. The presence of NH4+, as a percentage of available dissolved inorganic nitrogen, increased with distance from the Atchafalaya River, indicative of remineralization processes and NO3− reduction. Mean PO43− concentrations were often higher in the estuarine regions compared to the Atchafalaya River. During summer total suspended solid (TSS) concentrations increased with distance from the river mouth, suggesting a turbidity maximum. Highest chla concentrations were found in the bayous and shallow water bodies of the Terrebonne marshes, as were the lowest TSS concentrations. The low chla concentrations found in other areas of this study, despite high inorganic nutrient concentrations, suggest light limitation as the major control of phytoplankton growth. Salinity reached near seawater concentrations at the outer edge of the Atchafalaya River plume, but much lower salinities (<10 psu) were observed at all other regions. The Atchafalaya Delta estuarine complex buffers the impact of the Atchafalaya River on the Louisiana coastal shelf zone, with a 41% of 47% decrease in Atchafalaya River NO3− concentrations before reaching Gulf waters.

The 1994 experimental opening of the Bonnet Carre Spillway to divert Mississippi River water into Lake Pontchartrain, Louisiana

Abstract A diversion of Mississippi River water into Lake Pontchartrain, Louisiana, USA by way of the Bonnet Carre Spillway has been proposed as a restoration technique to help offset regional wetland loss. An experimental diversion of Mississippi River water into Lake Pontchartrain was carried out in April 1994 to monitor the fate of nutrients and sediments in the spillway and Lake Pontchartrain. Approximately 6.4×108 m3 of Mississippi River water was diverted into Lake Pontchartrain over 42 days. As water passed through the Bonnet Carre Spillway, there were reductions in total suspended sediment concentrations of 82–83%, nitrite+nitrate (NOx) of 28–42%, in total nitrogen (TN) of 26–30%, and in total phosphorus (TP) of 50–59%. 3.9±1.1 cm of accretion was measured in the spillway. Nutrient concentrations at the freshwater plume edge in Lake Pontchartrain compared to the Mississippi River were lower for NOx (44–81%), TN (37–57%), and TP (40–70%), and generally higher for organic nitrogen (−7–57%). The Si:N ratio generally increased and the N:P ratio decreased from the river to the plume edge. Nutrient stoichiometric ratios indicate water at the plume edge was not silicate limited, suggesting conditions favoring diatomic phytoplankton.

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.

Spatial and Temporal Variations in Nutrients and Water-Quality Parameters in the Mississippi River-Influenced Breton Sound Estuary

ABSTRACT Lundberg, C.J.; Lane, R.R., and Day, J.W., Jr., 2014. Spatial and temporal variations in nutrients and water-quality parameters in the Mississippi River-influenced Breton Sound estuary. The purpose of this study is to investigate the long-term temporal and spatial nutrient patterns in the Breton Sound estuary, an estuarine wetland complex in coastal Louisiana that is highly influenced by the Caernarvon river diversion. A water-quality data set spanning 8 years of monthly sampling was analyzed. Analysis of salinity mixing diagrams indicates the estuary to be a source of ammonium and chlorophyll a, and a sink for nitrate, total nitrogen, and total suspended sediments. The estuary served as either a source or sink for phosphate, total phosphorus, and silicate depending on season. The NOx loading rate ranged from 1.1 g N m−2 y−1 during fall to 4.9 g N m−2 y−1 during spring, with an overall mean of 3.5 g N m−2 y−1. Nitrate removal efficiency varied seasonally, with highest efficiency during the fall (98%), summer (92%), and spring (87%) and lowest during the winter (74%). There was an inverse relationship between nutrient removal efficiency and nutrient loading rate. The results of this study indicate that the estuary is effective in water-quality amelioration through nitrate removal.

Challenges in Collaborative Governance for Coastal Restoration: Lessons from the Caernarvon River Diversion in Louisiana

ABSTRACT In an effort to restore deteriorating coastal wetlands in Breton Sound, Louisiana, a diversion of Mississippi River water into the estuarine ecosystem has been operated at Caernarvon, Louisiana, since 1991. The diversion was implemented after a relatively long collaborative planning process beginning in the 1950s. The Caernarvon Interagency Advisory Committee, an official panel of stakeholders, considers scientific aspects of the freshwater inflow and stakeholder inputs in developing an operational plan, which internalizes stakeholder conflicts, while accomplishing goals of restoring the coastal ecosystem. Even though fishery representatives are committee members, local oyster fishers filed lawsuits in federal and state courts from 1994 through 2005, claiming damages to their oyster beds. These lawsuits were initially successful in state courts but were reversed by the Louisiana Supreme Court. The federal suits were unsuccessful. Following these lawsuits, voters in Louisiana in 2000 amended the State Constitution to protect coastal restoration projects against lawsuits reflecting increase in overall statewide support. Increasing scientific knowledge has contributed significantly to diversion operation. For better collaborative governance, efforts to increase common understanding among stakeholders will be needed, and a process to compensate interests of stakeholders suffering from impacts of restoration projects at an earlier stage should be institutionalized.

Modeling impacts of sea-level rise, oil price, and management strategy on the costs of sustaining Mississippi delta marshes with hydraulic dredging

Over 25% of Mississippi River delta plain (MRDP) wetlands were lost over the past century. There is currently a major effort to restore the MRDP focused on a 50-year time horizon, a period during which the energy system and climate will change dramatically. We used a calibrated MRDP marsh elevation model to assess the costs of hydraulic dredging to sustain wetlands from 2016 to 2066 and 2016 to 2100 under a range of scenarios for sea level rise, energy price, and management regimes. We developed a subroutine to simulate dredging costs based on the price of crude oil and a project efficiency factor. Crude oil prices were projected using forecasts from global energy models. The costs to sustain marsh between 2016 and 2100 changed from