Evaluation of Recycled Gypsum Application Dosages to Enhance the Water Infiltration Rate at Water Retention Ponds

The retention ponds play central role for efficient collection, removal, and drainage of rainfall runoff from surrounding streets to minimize flooding and damage. However, low percolating retention ponds may result in an extended ponding of water, abundance bacteria accumulations, mosquito breeding, environmental hazards, among others. The presence of clayey soils and soil sodicity reduces infiltration rates of retention ponds. To enhance the infiltration rate of ponds, it is proposed to use recycled gypsum (RG), as a soil amendment in retention ponds consisting of clayey soils to enhance low infiltration rates. To evaluate the influence of RG in clayey soils as a soil conditioner, the following approach was adopted in this study: (1) To evaluate influence of gypsum in enhancing infiltration rates, the double ring infiltrometer was utilized on the field and the falling head method in the laboratory. (2) To determine the optimum gypsum dosage, various application methods were employed to maximize benefits. (3) To identify the influence of soil and RG mineralogy, X-Ray Diffraction (XRD) technique was emplyed. (4) To identify and evaluate elements coming out at the bottom of the pond, different application cycles were employed and tests were performed using falling head permeability test protocol. (5) To evaluate the transportation of leaching metals to groundwater, the Risk-Based Corrective Action (RBCA) toolkit was used to identify movement of metals after RG application. (6) To simulate diffusion of RG in the water retention pond, a small-scale model was developed and tested. A total of four soils were collected from the city of El Paso, TX. One soil was collected from the “Upper Valley” region, and the other three were collected from local retention ponds in the area. These retention ponds are labeled for this study as Interchange, Westbound, Site pond, and Upper Valley soils based on their location. El Paso Water (EPW), had previously applied RG dosages to Westbound and Site pond before this study was conducted. The Upper Valley, Interchange, Westbound, and Site pond soils were classified as CL, CL, SM, and SM, respectively based on the Unified Soil Classification System (USCS). Both, Upper Valley and Interchange soil conditions were determined to be sodic and Westbound and Site soils as normal. The double ring infiltrometer was utilized at the Interchange pond to evaluate the influence of RG using different dosages and application methods. The gypsum application methods consisted of spreading gypsum on top of the soil surface (GSS) measured in tons/acre and mixing gypsum in distilled water (DDW), measured in percent of concentrated, saturated solution dose. Each application method was performed in triplicates leading to a total of 39 test. In the GSS application method, the initial (zero RG dosage) infiltration rates for the inner and outer ring were \u3c 0.1 in./h, and it was enhanced up to 0.35 in./h and 0.68 in./h for the inner and outer ring, respectively at 17.5 ton/acre gypsum dosage. Thus, enhancing the infiltration rate of the inner and outer ring by approximately 750 and 1,060 %, respectively. As for the DDW application method, the infiltration rate was linearly increased with dosage increase in increments of 25 %. The maximum average infiltration rate observed in the inner ring and outer ring was 0.40 in./h and 0.52 in./h, respectively. This method enhanced the infiltration rate approximately 550 % for the inner ring and 1,230 % for the outer ring. The falling head test was used to determine the saturated hydraulic conductivity (Ksat) in response to different solutions. These samples were prepared at approximately 90 ± 2 lb/ft3 dry density and infiltration solutions were prepared by dissolving gypsum in deionized water (DI) and in collected water from the Westbound pond identified as pond water (PW). These saturated solutions mixed in DI and PW are denoted as DDI100 and DPW100. Subsequently, DDI100 and DPW100 were diluted with DI and PW at different percentages by volume. For example, a dose of DD75 means that 75 % DDI100 and 25 % DI was mixed. Similarly, for a DPW25 dose, 25 % of DPW100 was mixed with 75 % PW. Falling head permeability tests using DDI and DPW method showed a linear trend in all four soils. Interchange and Upper Valley soils Ksat value increased for both application methods, as higher concentrated gypsum solution dosages were applied. However, Ksat values of Interchange soil were higher than Upper Valley even though both soils were characterized as CL in sodic conditions. This phenomenon can be caused by factors such as lower: cation exchange capacity, specific surface area, plastic index, among others. (Abstract shortened by ProQuest.