Date of Award
Master of Science
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 < 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. For sandy soils (Westbound and Site pond soils), gypsum applications had minimal effect on Ksat values as compared to Upper Valley, and Interchange soils due to previous gypsum application and sandy soils do not have a high negatively net charge in comparison to clayey soils. The Ksat incremental value for the DDI and DPW methods in all four soils with increasing of RG dosages were:
Upper Valley Interchange Westbound Site
DDI: 693 %
DPW: 443 % DDI: 68 %
DPW: 215 % DDI: 1 %
DPW: 6 % DDI: 6 %
DPW: 8 %
The gypsum application method was studied using the falling head tests with leaching solutions at incrementally increased dosages as described in the previous paragraph. Additional gypsum application methods were employed to compare their influence on the saturated hydraulic conductivity obtained from DDI and DPW methods. Interchange soil was selected for analysis because the site has not been treated with gypsum and is more accessible than the Upper Valley soil in terms of higher infiltration rates and fewer complications in saturating soil samples prior to permeability measurements.
The gypsum application methods employed to determine the hydraulic conductivity were:
1. Gypsum dissolved in deionized water (DDI)
2. Gypsum dissolved in pond water (DPW)
3. Mixed gypsum in soil by weight at 10 %, 20 %, 30 %, 40 %, and 50 % (MGS)
4. Gypsum spread at the soil surface at the rate of 3.5, 6.9, 10.4, 13.9, 17.3, 20.8, 24.3 tons/acre (GSS).
The Ksat values in all application methods linearly incremented as higher RG applications were employed. The incremental Ksat value in (%) for the interchange soil for the different gypsum application approach was:
DDI: 68 %
DPW: 215 %
MGS: 337 %
GSS: 32 %
It was noticed that MGS method provided a higher incremental in Ksat as compared to other methods. Although, MGS Ksat value grew much higher due to an overall average dry density reduction of 27 %, allowing more conductive paths for water to flow through. GSS delivered the lowest increase in Ksat values. Since it is to expected that calcium would increase the permeability of clayey soils, GSS application approach did not have enough time for calcium to solubilize in DI water to effectively provide the calcium needed for clayey particles to flocculate and provide more spaces for water to flow through.
To evaluate contaminants that may leach with use of RG, the leached solutions were collected after â??continuous pondingâ?? application and tested to simulate retention pond experiences. Discharged leachate outflow was collected in 3 cycles: 30 mL, 80 mL, and 130 mL, and specimens were sliced in three equal parts (top, middle, and bottom) after the completion of each cycle. Collected solutions at the end of soil column, top, and bottom sliced sections were analyzed in the Inductively Coupled Plasma Spectrophotometer (ICP). As for DI applications in the upper valley soil, Ca2+ concentrations in collected, bottom and top sections were low (<200 mg/L) and low sodium concentrations as (< 600 mg/L). After RG applications, Ca2+ concentrations for the top and bottom sections increased to 350 mg/L, and sodium concentrations increased to more than 700 mg/L for both DDI100 and DPW100 application methods. This experiment showed higher calcium concentration in the top and bottom sections and higher sodium concentrations of the collected water as opposed to DI applications only. Also, the electric conductivity (EC) was measured for the collected water for DI, DPW100, and DDI100 in cycles of 30 mL, 80 mL, and 130 mL Once DDI100 and DPW100 were applied, EC values approximately doubled compared to EC values obtained from DI applications. Also, as higher leaching cycles were applied, EC values decreased in each application method. For example, in DDI100 applications, at cycle 30 mL; the EC conductivity measured was 4.76 mS/cm, at 80 mL the EC value decreased to 4.6 mS/cm, and for 130 mL it was reduced to 4.15 mS/cm.
To identify the effects of RG on Ksat in highly compacted clays, the interchange soil was used to represent the low plastic clay (CL) at the bottom of the pond. A total of 9 soil specimens were compacted in a 4 in. diameter by 4.6 in. in height mold as per ASTM D698-12. The mold was coated with Rust-Oleum rust and corrosion protection before its usage. The 125 pcf moist density was achieved by compacting the specimen to an 18.5 % moisture content based on the in-situ density of the Interchange soil, subsequently saturated for approximately 30 days. The Ksat value was measured with the leaching solutions: DI water, DDI100, and DPW100. Permeability and leaching experiment was executed simultaneously in cycles of 48, 120 and 168 hours. Once testing was completed, the soil was sliced in three equal parts for bound cation analysis. Both collected water, and top, middle and bottom slices were analyzed for cations and heavy metals in the ICP spectrophotometer. Compared to DI water leaching solution, DDI100 and DPW100 enhanced the saturated hydraulic conductivity by an approximate average of 19 % and 29 %, respectively.
As part of analyzing the saturated hydraulic conductivity, the collected water and sliced sections (top, bottom, and bottom) were chemically analyzed for cations subsequently after the completion of the falling head permeability test. Similar to the "low density" leaching results, the Ca2+ concentration for DI was relatively smaller as compared to DDI100 and DPW100 for the top, middle and bottom sections. The initial average Ca2+ concentrations for the three sections was approximately 300 mg/L. Once DDI100 and DPW100 were leached through the specimens, the Ca2+ concentration increased to approximately in the range of 450 mg/L to 600 mg/L.
A theoretical metal leaching analysis was employed using the RBCA Toolkit of Chemical Releases software. It was conducted to assess the effect of simulated RG dosages in a retention pond and analyzing the migration of gypsum trace elements (arsenic, cadmium, chromium, lead, manganese, nickel, and zinc). Representative source media constituent of concern (COC) concentration was entered directly by considering the bound cations extracted from Westbound soil that have gypsum dosages in mg/kg. In the model, two off-site receptors or water wells were assumed at a distance from the source (water table under the pond soil column) of 200 and 1,000 feet. Based on the concentration dosages in the source, time, and soil permeability, an analysis was performed to determine individual COC concentrations in groundwater at x distance with respect to time. The results for all trace elements exhibited a similar trend. For example, the arsenic concentration after 6 weeks reached 6 x 10-4 mg/L. Two considerations or trends were observed, one at â??one-yearâ?? trend and the other one as a â??steady-stateâ?? condition. Constant arsenic concentration applications for one year resemble the â??steady-stateâ?? condition and reduces to the offset of 200 feet away from the source to approximately 4.0 x 10-4 mg/L, reducing exponentially to close to 0.0 mg/L at an offset of 1,000 ft. The trend representing "one-year" represents a single RG application. Since leaching water would leach arsenic concentrations from the soil, the total arsenic would be thoroughly diluted, having zero concentration reaching to offset 200 and 1,000 ft.
Several factors contribute to the benefits of implementing RG in retention ponds. This incorporation leads to increase in the infiltration rate of clayey soils, eco-friendly, favorable to the economy, and is sustainable for the city of El Paso. The life cycle assessment for a typical drywall board is derived into paths, either material lasts to its end of life and is sent to a landfill, or material is recycled. In this analysis, a summary of environmental concerns was analyzed based on every stage of the process throughout its service life. As for the development of the drywall, its composition is made by inputs such as mining gypsum, FGD gypsum from manufacturing plants, collect facing paper, and additive during production. Although the transportation and other factors enhance CO2 emissions, the drywall recycling allows reuse of 12 % rejected drywall material and 64 % gypsum board scraps. If this 76 % is not recycled, it would be disposed of in landfills, requiring more resources for disposal and being environmental hazard (like generation of hydrogen sulfide in the landfill).
It is recommended to EPW to:
Dissolve RG in water as the most suitable application method.
Implement an aeration system in retention ponds to reduce the probability of generating toxic gases (H2S) when RG is applied. (see Appendix H for proposed market mixers)
For future RG applications, it is recommended to grind particles < 0.425 mm for faster dissolution rate.
Recommend RG supplier to remove as much as possible cellulose material.
Maintain documentation or a log of RG application for future leaching analysis.
Apply approximately 2.5 g/L of RG in water.
Since Interchange pond is generally at low water levels due to its high area, it is recommended to mix RG in soil by tillage.
Received from ProQuest
Jorge Luis Navarrete
Navarrete, Jorge Luis, "Evaluation Of Recycled Gypsum Application Dosages To Enhance The Water Infiltration Rate At Water Retention Ponds" (2018). Open Access Theses & Dissertations. 130.