Water quality analysis of a freshwater diversion at Caernarvon, Louisiana

Since 1991, Mississippi River water has been diverted at Caernarvon, Louisi ana, into Breton Sound estuary. Breton Sound estuary encompasses 1100 km(2) of fresh and brackish, rapidly subsiding wetlands. Nitrite + nitrate, tota l Kjeldahl nitrogen, ammonium, total phosphorus, total suspended sediments, and salinity concentrations were monitored at seven locations in Breton So und from 1988 to 1994. Statistical analysis of the data indicated decreased total Kjeldahl nitrogen with associated decrease in total nitrogen, and de creased 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 t hese 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 effi ciencies for these constituents (nitrite + nitrate 88-97%; total nitrogen 3 2-57%; total phosphorus 0-46%). The low impact of the diversion on water qu ality in the Breton Sound estuary, along with assimilation of TSS over a ve ry short distance, suggests that more water may be introduced into the estu ary without detrimental affects. This would be necessary if freshwater dive rsions are to be used to distribute nutrients and sediments into the lower reaches of the estuary, in an effort to compensate for relative sea-level r ise, and reverse the current trend of rapid loss of wetlands in coastal Lou isiana.