National Institute for Water Resources, Water Resources Research Institute Program
3/1/03 - 2/28/06
Mobility of viruses, bacteria, and other wastewater contaminants (including hormones) in subsurface media and their arrival in groundwater can have public health as well as aesthetic concerns. The sources of human and animal pathogens that may impact groundwater quality include land-applied sludge, manure, and wastewater; storage sites for manure; cesspools and septic tanks; leaky sewers; lagoons for the storage of human and animal wastes; barnyards; and feral animals. Wastewater and manure are also typical sources of hormones and other pharmaceutical compounds. Potential exists for pathogens and hormones/pharmaceuticals and other wastewater contaminants to leach through the soils and to finally reach the drinking water aquifers.
Generally, it is believed that the physical and chemical characteristics of the soil and to some extent the characteristics of the pathogens (as well as hormones) affect transport in subsurface media. If a soil is aggregated, it provides preferential pathways for pathogen- or hormone/pharmaceutical-containing water to pass through the topsoil. Fractures and cracks in subsoil accelerate the movement of this water. The clay and organic matter content of the soil affect the sorption of pathogens, hormones, and pharmaceuticals, while soil pH and the isoelectric point of the pathogens affect their attachment. Also, it is believed that the mineral oxides provide positively charged sites to retain these pathogens in the soil. The degree of saturation of the subsoil media may have some effect on the mobility of pathogens. If the water content is reduced, it is hypothesized that the travel path will be longer and the water film thickness with be smaller. A smaller thickness of the water film around the particles allows greater attachment potential of the pathogens to the particles.
An initial evaluation of a local soil (a tropical Oxisol) showed that it has high potential for retaining a large number of bacteria and viruses in the top three inches of packed soil columns. Oxisols contain a large percentage of clay-size particles and contain significant amounts of the oxides of iron, aluminum, and manganese. For this soil, batch-equilibration sorption experiments conducted for bacteriophage showed a high sorption distribution coefficient. As for bacterial sorption, although methodological problems caused some difficulties in quantitation, the initial results showed higher retention. Although this result has aroused widespread interest locally on the reuse of effluent on land directly overlying potable water aquifers, we strongly believe that additional laboratory and field evaluations are essential prior to undertaking large-scale land application efforts. More remains to be done in characterizing various soils in terms of their physical and chemical properties where land application of wastewater is being or is planned to be practiced. Most soils in temperate climates have fixed charges, whereas many soils in the tropics exhibit variable charges. In addition, quantification of attachment and detachment of the bacteria and viruses through batch-equilibration sorption tests or through flow-through column experiments needs to be evaluated.
The initial goal of this research was to evaluate the transport of pathogens and pharmaceutical compounds as influenced by soil properties, especially under Hawaii conditions. The planned study was proposed to be carried out over a two-year funding cycle. Since funding was provided for just one year, only the transport of selected wastewater contaminants and hormones was considered. Thus the objectives were modified to address the following:
1. How would the wastewater contaminants and hormones move through Hawaii soils?
2. Can the soil properties be manipulated by amendments to affect contaminant transport?
3. Can the existing models be used to quantify contaminant transport in the subsurface?