Erosion Control and Removal of Suspended Soil Particles in Ponds: Evaluation of Geofabric Liners and Chemical Coagulants

Abstract

In an experiment conducted at the E. W. Shell Fisheries Center, Auburn, Alabama, three ponds each were lined with a permeable geotextile with 0.090-mm opening sizes, a permeable geotextile with 0.355-mm opening sizes, or served as unlined, controls. During the 2-yr study, geotextile liners did not tear or decay. This prevented erosion of the bottom of the ponds stocked with a high density of channel catfish and aerated nightly. The liners tended to float up in the water column; the liner with smaller opening sizes floated in all three ponds, while the other liner floated in only one pond. Fish production, survival, and feed conversion ratio did not differ (P > 0.05) among the control and lined ponds or between liner types. Water quality variables often differed between lined ponds and control ponds. The greatest difference was higher phytoplankton abundance in the lined ponds―especially in ponds lined with the less permeable geotextile – than in the control. This difference was attributed to differences in phosphorus uptake by bottom soils related to the liners and their permeability. However, in spite of more phytoplankton growth in lined ponds, the control ponds tended to have higher turbidity because of erosion of bottoms by aeration. To investigate phosphorus uptake rate by soil through the geotextiles, six kinds of geotextile liners with apparent opening sizes ranging from 0.090 mm to 0.84 mm were installed in separate soil from water in aquaria. Water was treated with 0.75 mg/L of phosphorus from monopotassium phosphate. Phosphorus concentration did not decline in aquaria without soil, and all liners reduced the amount of phosphorus removed by the soil. Phosphorus removal by soil did not differ among liners with opening size < 0.200, but these liners interfered less with soil phosphorus uptake than did the liner with 0.090-mm opening size. Final phosphorus concentrations were 0.089 mg/L in the unlined aquaria with soil, 0.397 mg/L for the liner with 0.090-mm openings, and 0.157 to 0.202 mg/L for the liners with larger openings. Geotextiles can reduce erosion resulting in less turbidity in water. However, fine particles settle very slowly and may remain suspended almost indefinitely restricting light penetration in the water column and reducing primary productivity. Particles of three soils – each containing a different type of clay – settled at different rates. Nevertheless, particles tended to settle faster as concentration of total dissolved solids (TDS) increased in diluted seawater. In freshwater, total hardness concentration increased as TDS concentration rose. Several chemical coagulants were tested for their ability to remove suspended soil particles. Potassium and sodium chloride were comparatively ineffective for turbidity removal. Calcium sulfate was more effective than magnesium sulfate in removing turbidity. Aluminum chloride and sulfate tended to perform better than ferrous sulfate and ferric chloride. Although there were some differences in effectiveness of coagulants among the types of soils, calcium sulfate (gypsum), aluminum sulfate (alum), and aluminum chloride appear to have the greatest potential for use in ponds. Aluminum compounds neutralize alkalinity and cause a decrease in pH. Treatment rates (mg/L) for aluminum sulfate and aluminum chloride should not exceed the total alkalinity (mg/L as CaCO3) to avoid low pH. Knowledge of conductivity, total alkalinity, total hardness, and type of clay mineral suspended in water can be useful in selecting a suitable coagulant.