Investigators: Chittaranjan Ray, U.H. Civil Engineering, Samir A. El-Swaify, U.H. Soil Science
Problem and research objectives
Application of high-molecular-weight anionic polymers is a fairly new technique being used for soil erosion control on land under furrow, and to a limited extent sprinkler irrigation. It has been demonstrated that more than 95% of sediment loss from land under furrow irrigation can be prevented by applying a high-molecular-weight polyacrylamide (PAM) at a concentration of 10 mg/l. in irrigation water. Use of PAM also has significant potential to reduce soil loss at construction sites or any place where soil is disturbed. While performing erosion-control experiments our research team discovered that PAM also enhances the water-infiltration capacity of soils. Enhanced infiltration in soils treated with wastewater or biosolids could potentially pose a health risk due to the acceleration of pathogenic organisms through the soil to the groundwater. However, the effect of polymer addition on pathogen transport has not been investigated. Soil pH, clay and organic matter content, degree of water saturation, and other environmental and geologic factors are expected to impact the transport behavior of pathogenic organisms.
The objectives of this research are to examine the transport of bacteria, bacteriophage (bacterial viruses), in polymer-amended agricultural soils, and to compare transport behavior in polymer amended soils and untreated soil. The following describes our activities to date.
MethodologyOur first step was to conduct an initial screening of several polymers for their capacity to reduce erosion and enhance infiltration in several Hawaii soils. After detemining the best performing polymer we conducted breakthrough experiments for bromide in packed sand columns under various degrees of water saturation. Following that, soils were obtained from agricultural land on Oahu where wastewater and polymers may be applied some day. The soils were air dried, passed through a 4-mm sieve, and packed to field bulk densities in columns between 10 and 22.5 cm. long. The columns were saturated with a leaching solution. In a series of tests water was applied at 5 cm/h (similar to an intense storm) for various periods of time . Once steady-state flow was established, bromide was injected at the top of the column either as a pulse or continuously in the leaching solution. The bromide breakthrough data allowed us to estimate solute transport parameters. In subsequent experiments, a laboratory strain of Escherichia coli, sewage effluent, bacteriophage (MS-2), and enterococcus bacteria were added to the leaching solution. Initially the organisms were added as a single pulse at the start of the experiment, breakthrough was monitored as a function of time. The feed solution had cell counts for bacteria ranged from 5 to 8 log orders (100,000 to 100 million) and for the phage 7 logs (10,000,000)/ml. The leachate was collected until 10+ pore volumes of solution had passed through the column. In all cases, the leachate contained no bacteria or virus. In subsequent experiments, either MS-2 phage or bacteria were injected continuously along with the leaching solution. Leaching continued until at least 20 pore volumes of solution passed through the soil column no breakthrough.
Principal findings and significance
1) Enhancement of infiltration
The polymer screening study involving four Hawaii soils found that with the exception of one soil (a Vertisol), the polymer worked well in preventing soil loss and enhancing infiltration. For the Wahiawa Oxisol that we examined, nearly 98% of applied rainfall infiltrated after a single polymer application of 10 kg/ha; the infiltration rate was 25% to 40% greater than that for the untreated soil. It was concluded that polymer amendment of Hawaii soils could significantly enhance infiltration and recharge.
2) Microbial Transport
We have not yet conducted microbial transport column experiments on polymer treated soil. We have completed experiments on untreated sand and soil columns and we are currently conducting additional tests on untreated soil columns. E. Coli in sand column
Initial results of E. coli transport studies in silica sand columns indicate that there was significant retention of this bacteria in the sand. While the input concentration was on the order of 8 logs/ml (10 ml applied), the bacteria count at the bottom of a 15-cm sand column was reduced to a maximum of 4 log bacteria/ml after 2.7 pore volumes were passed through the column. Peak concentration of both bromide and bacteria occurred at the same time indicating that the bacteria's trnsport was not merely delayed but arrested by the sand.
Bacteria & Phage in soil column
A Wahiawa Oxisol soil which showed the best promise in the screeening tests was used later for both bacteria and phage leaching experiments. This is a very common soil on Hawaii's agricultural lands. A shorter (4-inch) soil column was used. A solution of bromide at a concentration of 5 mg/l and E. coli at 1.96 x 107 colony-forming units (CFU)/ml was continuously applied to the top of the soil column at a rate of 5 cm/h. The experiment continued until 6 pore volumes of water passed through the column. The bromide concentration reached its peak (original concentration) after 3.4 pore volumes of leaching solution had passed through the column. The peak concentration of bacteria in the leachate was less than 1 log order/ml but still rising at the end of the experiment although at very low concentration. It was concluded that a longer experiment was needed.
A second experiment was run under similar conditions but for a longer time. Approximately 20 pore volumes of leaching solution passed through the soil column. The feed concentration of E. coli was 1.16 x 107 CFU. An average of less than one log of E. coli/ml of leachate was discovered between 4.65 and 20 pore volumes. The sorbed bacteria concentration from samples collected from the soil column was on the order of 107 CFU/g of dry soil.
A bacteriophage, MS-2, was used in the third experiment. The hydraulic conditions for the soil column remained the same. The experiment was run for 15 pore volumes. The bacteriophage stock solution had to be replenished once during the experiment. The initial concentration of the phage solution was 107 plaque-forming units (PFU)/ml and at the end of the experiment it was 106 PFU/ml. No bacteriophage was detected in the leachate throughout the experiment. The average number of phage adsorbed to the middle portion of the soil column was 108 PFU/g of dry soil, and that at the bottom portion of the soil column was 105 PFU/g. Unsaturated conditions with low pH as well as high iron and clay content helped attenuate the bacteria and phage in the soil column.
Presently a longer experiment using MS-2 bacteriophage in untreated soil is underway. This experiment will run 100 pore volumes of stock solution through a 4-inch soil column. If breakthrough is obtained, PAM and lime will be applied to the soil column, and the experiment will be repeated. The same experiment will be repeated for bacteria and microspheres.
Our inability to obtain a breakthrough of phage in these column tests indicates that the tested soil type is very efficient at attenuating the transport of microbial contaminants and that therefore the reuse of effluent in Hawaii probably would not have adverse impacts on the groundwater.