Linda F. Benedict, Arceneaux, Allen E., Bengtson, Richard L., Selim, M., Jeong, Changyoon
H. Magdi Selim, Chang Yoon Jeong, Allen E. Arceneaux and Richard L. Bengtson
The quality of water resources is a major concern in Louisiana and nationally. Surface and subsurface water quality depends in large part on human activities in the surrounding areas. Management strategies that reduce the potential for contamination are the key to quality of water resources. Bayou Wikoff in south Louisiana (Figure 1) was chosen by the Louisiana Department of Environmental Quality (DEQ) for intensive watershed-level monitoring and best management strategy implementation. The implementation process will allow DEQ to determine whether the management strategies being recommended for nonpoint-source pollution reduction do, in fact, reduce the pollutant load to the water body by 30 percent to 50 percent. The major goal of this water quality study is to quantify control measures for reducing contamination of sediment, nitrogen, phosphorus and dissolved organic carbon.
The project area is the Bayou Wikoff sub-watershed, which is representative of the type of land use that exists within the upper portion of the Bayou Plaquemine Brule watershed (Figure 1). The major land-use is agricultural production, specifically sugarcane and pasture. A total of seven sites were instrumented and include pastures sites, sugarcane sites and a control or pristine site.
The pasture sites include one continuous grazing site, which represents a traditionally managed pasture at maximum recommended stocking rates, supplemental feeding, winter seeding and fertilization, and minimum tillage. The second pasture site consists of a deferred rotational grazing system wherein duration, intensity and frequency of grazing were managed to enhance nutrient cycling. Therefore, we regard this management strategy as minimum input and reduced grazing pressure compared to the higher stocked continuous grazing site.
The sugarcane sites were side-by-side where one tract was selected for implementation of the standard practice of burning prior to or following harvest. The second site was a no-burn strategy to achieve nonpoint-source reductions in sediments and nutrients.
In addition, we carried out in-stream monitoring of water quality in Bayou Wikoff at two locations – an upstream location not influenced by the various treatments and a downstream location where the cumulative effects of all management treatments in Bayou Wikoff could be assessed.
To quantify the impact of best management strategies in the watershed, a minimally disturbed site was identified as the pristine site to obtain an expected level of natural background nonpoint-source loading. This site was identified as a control. It was a minimum-input tree farm wherein sapling trees of various species were grown within dense indigenous grass vegetation and heavy mulch applications at the tree base. The grassed areas are not grazed and only infrequently mowed. Effluent samples from this site and all instrumented sampling sites were activated by rainfall that produced runoff events.
Results of sediments or total solids in T the effluent from the rotational grazing site were significantly lower compared to the continuous grazing site. A reduction of total solids in the effluent of 79 percent was achieved when rotational rather than continuous grazing was the farming practice. Sediment losses or total solids losses from pastures sites were calculated based on runoff volume associated with each runoff event over the drainage area from each field. Losses of total solids at the pasture sites were nearly 40 percent higher under continuous grazing than under rotational grazing. The concentrations of suspended solids were not significantly different for the two grazing sites. However, the concentration of dissolved solids showed significant differ ence in two pasture lands. The amount of losses of suspended and dissolved solids exhibited similar trends with dissolved concentrations. Moreover, the turbidity from the two pasture lands showed a similar pattern with the results of the suspended solids.
The concentration of total phosphorus was significantly lower for rotational grazing than for continuous grazing. The dissolved phosphorus followed a similar trend. Dissolved phosphorus was significantly lower when rotational grazing was the management practice. Therefore, the loss of phosphorus in the effluent was 74 percent higher under continuous grazing than under rotational grazing.
The pristine site was considered a control site because management operations are restricted to mowing, mulching and clearing of drains throughout the year. Based on results from the pristine site, there is strong evidence that the presence of grass cover on the soil surface throughout the year minimized sediment losses and erosion. Based on mean comparison of total solids from the pristine site, it was not significantly lower from the continuous grazing site. Moreover, total phosphorus losses from the pristine site were significantly lower than those from the rotational and continuous grazing pasture sites.
Downstream and Upstream Sites
The concentrations of total solids from the downstream site of Bayou Wikoff were significantly higher than from upstream. Moreover, suspended, dissolved and volatile solids as a fraction of the total solids also showed higher concentrations in water samples from the downstream location when compared to concentrations from the upstream site.
H. Magdi Selim, George and Mildred L. Caldwell Professor of Soil Science; Chang Yoon Jeong and Allen E. Arceneaux, both Research Associates, School of Plant, Environmental & Soil Sciences; and Richard L. Bengtson, Professor,Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.
This study was funded in part by the Louisiana Department of Environmental Quality.
(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)