Enhanced Techniques for Determining Changes to Soils Receiving Wastewater Irrigation for Over Forty Years
Open Access
- Author:
- Walker, Charles William
- Graduate Program:
- Soil Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 28, 2006
- Committee Members:
- Hangsheng Lin, Committee Chair/Co-Chair
Daniel Dale Fritton, Committee Member
Maxim J Schlossberg, Committee Member
Gary Walter Petersen, Committee Member
Richard Rudolph Parizek, Committee Member - Keywords:
- Saturated Hydraulic Conductivity
Wastewater Irrigation
Hydropedology
Landscape Hydrology
Tension Infiltrometer - Abstract:
- It is the goal of the present study to use the framework set out by the emerging hydropedology concepts and techniques to better understand a real world practice, wastewater irrigation. The objectives are twofold, 1) to evaluate and modify selected hydraulic conductivity measurement techniques to enhance their reliability and accuracy; and 2) to comprehensively evaluate the changes in soil properties after receiving over forty years of wastewater irrigation, including the use of improved methodologies addressed in the first objective. The tension infiltrometer is a standard tool for measuring near-saturated soil hydraulic properties. We examined the dynamics of the supply tension at the interface between the tension infiltrometer and the measured soil using a pressure transducer under different soil conditions and raised some cautions needed for proper use of this standard device. Infiltration experiments were conducted on a tension table, a large sand column, and in two field soils of contrasting textures and structures to test the performance of the standard two-piece infiltrometer. Results showed that during high flow rates (> 200 cm3 min-1) the tension at the infiltrometer/soil interface started to deviate by as much as 15 mm from the desired tension. However, during field experiments the high flow rates were not experienced, and thus no deviation was observed between the pre-set desired tension and the actual measured tension at the infiltrometer/soil interface. To alleviate the problem of tension deviation under high flow, the water supply tubing and fitting diameters of the standard infiltrometer were successfully increased to yield a higher flow rate (~300-400 cm3 min-1) without elevated tensions. The constant head method for determining saturated hydraulic conductivity is a classical laboratory method for measuring the soil’s ability to conduct water. One error commonly associated with this technique occurs when there is flow between the edge of the soil sample and the cylindrical ring holding the soil sample. We developed a method to minimize the effects of this artificial boundary flow. A new simple permeameter was constructed to separate flow between the outer and inner portions of the soil core. Intact core samples from the surface and subsurface Hagerstown silt loam series were analyzed to quantify the edge flow phenomenon. Outer saturated hydraulic conductivity (associated with possible edge flow) was found to be significantly higher than the inner saturated hydraulic conductivity (p = 0.038, n = 153). The A-horizon surface samples were found to have significant edge flow (n=110, p =0.049), whereas the subsurface soil samples did not experience significant edge flow because of possible clay expansion. In addition, brilliant blue dye was used to visually confirm the edge flow phenomenon. This study also investigated the length of saturation time needed to saturate the soil cores, and found that the heavier textured cores should be saturated for at least a week. A long term experiment was conducted for eight days to determine if and when the outflow volume reaches steady-state. The results indicated that, although the core appeared to be at steady-state during short time periods, true steady-state conditions were not achieved until approximately five days into the experiment. Furthermore, the new permeameter’s results were compared with in situ hydraulic conductivity methods. Taken altogether, these results suggest that the hydraulic conductivity values from the inner portion of the soil core are more comparable to the in situ tension and double ring infiltrometer conductivity values observed in the field. For over 40 years, The Pennsylvania State University (PSU) has irrigated its wastewater onto both cropped and forested lands. While this method of wastewater disposal has gained popularity in water-deficit regions, it is not widely used in areas that have a surplus of water. Despite local weather conditions, PSU sprays two inches of wastewater a week. This irrigation, combined with the natural precipitation, amount to approximately 140 inches of water per year, which is equivalent to tropical rainfall. The objective of this study was to investigate the morphological and functional changes in soils as a result of this increased water load. Previous studies conducted at this site provided an estimate of the original soil properties. Soil morphological parameters, such as structure, horizonation and redoximorphic features, were evaluated from the soil cores and in situ soil pits. In addition, soil functional parameters, such as saturated hydraulic conductivity, bulk density, texture, organic matter content, and pH, were evaluated to determine the longevity of the wastewater irrigation system. Results indicate that the soils are experiencing periods of saturation and local erosion, which are explained by redoximorphic features and over thickened A-horizons found on the site. Although redoximorphic features were found to experience a significant increase in size since the last description in 1978, they did not increase in frequency. The site was also evaluated based on three different landscape positions: summit, side-slope, and depression. These positions were found to be statistically different when describing morphological features (i.e., manganese coating percentage, A-horizon depth, soil structure) and physical features (i.e., bulk density). As a result of prolonged irrigation, soil functionality has also changed. Visual observations of increased runoff amounts are supported by laboratory findings of reduced saturated hydraulic conductivity in some regions of the study area. Collectively, these findings indicate that the summit areas may be the cause of excess runoff, having the lowest hydraulic conductivity values and most eroded A-horizon. Conversely, while the depressional areas are the most saturated, a result of receiving extra runon from the summit positions, laboratory hydraulic conductivity results indicate these depressions can transmit the greatest amount of water. We therefore propose remediation to increase the hydraulic conductivity of the summit areas in order to keep this area sustainable for future years to come.