| Abstract: | Surface water quality impairment in agricultural watersheds is a major environmental
concern in the United States. To assess seasonal and spatial variability of surface water
quality and identify factors associated with surface water quality variability, we monitored surface
water quality at seven locations in Bayou Plaquemine Brule Watershed in Louisiana twice
monthly from March of 2002 to February of 2008 and performed multivariate analyses of the
dataset. Using the Soil and Water Assessment Tool (SWAT) model, we identified critical areas
of nonpoint source pollution in the watershed. While temperature, turbidity, dissolved oxygen
(DO), conductivity and pH were determined in the field using YSI Sonde (YSI Incorporated,
Yellow Springs, Ohio), surface water samples were analyzed for total nitrogen (TN), total
phosphorus (TP), nitrate/nitrite-N (NO3/NO2-N), soluble reactive phosphate (SRP), total
suspended solids (TSS), and five-day biological oxygen demand (BOD5) in laboratory. The
monthly water quality sampling included a regular sampling and an after-rain-event sampling.
Average DO for the summer months, March through November, was 4.91 ± 0.08 mg L–1
(4.91 ± 0.08 ppm), while average DO for the winter months, December through February,
was 8.32 ± 0.12 mg L–1 (8.32 ± 0.12 ppm). Dissolved oxygen was negatively correlated with
TN (r = –0.22, p ≤ 0.001), SRP (r = –0.17, p ≤ 0.001), TP (r = –0.17, p ≤ 0.001), BOD5
(r = –0.25, p ≤ 0.001), and surface water temperature (r = –0.70, p ≤ 0.001). Turbidity was
strongly correlated with TSS (r = 0.59, p ≤ 0.001), suggesting that most turbidity in the water
body comes from suspended solids. Similarly BOD5 was significantly positively correlated
with TN (r = 0.43, p ≤ 0.001), NO3/NO2-N (r = 0.26, p ≤ 0.001), TP (r = 0.25, p ≤ 0.001),
and SRP (r = 0.18, p ≤ 0.001). Results of factor analyses showed sediment, phosphorus (P),
nitrogen (N), surface water temperature, dissolved solids, and acidity/alkalinity as the most
important factors associated with surface water quality variability in this watershed. Although
relatively higher concentrations of sediments, TP, and TN were observed in the upper reaches
of the watershed based on water quality monitoring, the SWAT simulation results showed
the critical nonpoint source pollution areas of sediment, P, and N in the lower reaches of
the watershed. Lower reaches of the watershed have mainly rice and crawfish production,
while the upper reaches include primarily sugarcane, pasturelands, and soybean production.
Information on seasonal variability of surface water quality, factors associated with surface
water quality variability, and the critical areas for nonpoint source pollution will be valuable
inputs for developing a watershed management plan for effective nonpoint source pollution
control in an agricultural watershed. |