Biosolids recycling : a proposed methodology for the assessment of the impact on groundwater

A groundwater risk assessment protocol is needed for land restoration schemes using recycled biosolids. A hydrogeological risk assessment for the Darnconner site in East Ayrshire [NS5723 to NS5823] has been used as a case study to develop the protocol. The proposed outline for developing the protocol included the following components: 1. Gather available geological information for the site and environs from 1: 50 000 scale geological maps and more detailed information where available. 2. Interrogate the BGS borehole database (Wellmaster) to establish existing groundwater users in the vicinity and available information regarding depth to water, water quality, etc. 3. Identify the catchment Baseflow Index (HOST) to establish the degree to which surface water courses are groundwater dependent. 4. Bring the three strands of information together to create a preliminary conceptual groundwater flow model for the vicinity of the site. This is presented as a three dimensional schematic supported by cross sections if details are available. 5. Identify pathways from the site to the water table, such as former adits, local faulting etc., in addition to likelihood of any intergranular ingress. 6. Access BGS HiRES airborne geophysics images to see if existing pollution plumes derive from or cut across the recycling sites. 7. Evaluate the potential risk to groundwater from the site and likely flow path any contaminated groundwater may take, including emergence to surface water. 8. Advise on any further work that may be required, e.g. drilling and monitoring. The time estimate for each assessment was aimed at four man days (including Coal Authority input to access mine abandonment plans for former collieries in the vicinity, and time for accessing the HiRES data where appropriate, to evaluate pollution egress from treated sites), plus the walkover site of all four sites, a further one day. Bacteria are largely attached to soil particles, are not easily mobile and die-off is rapid in the soil profile. Bacterial risk to groundwater is, therefore, low unless there is direct transport from the soil via a fracture or existing mine adit to the water table. The risk from metal contamination of groundwater is also low as metals are bound to the sludge organic matter and the receiving soil provided the pH is maintained at a near neutral environment. The main potential pollution risk from biosolids is nitrate derived from the oxidation of ammonium and nitrite species leaching from the topsoil. Risk assessment hinges on the potential for transport of nitrogen species to groundwater. A scorecard system to assess the nature of the pathway and its potential to transport nitrogen rich infiltration to the water table is presented. The use of sewage sludge as a primer for vegetative growth and as a humic foundation for soil development at mine sites is not new. It was common practice in the 1980s and 1990s to mix biosolids with the more shaley rock wastes from colliery and opencast sites to attempt to recreate a soil cover. The technique was, for example, applied repeatedly and successfully during post-closure procedures of numerous opencast sites in South Wales. There are no recorded instances of bacterial, metal or nitrogen contamination of local groundwaters, although background levels of all three potential contaminant groups were likely to have been elevated during the initial post-closure period due to intensive mining and industrial activity taking place at that time. Detailed descriptions of this work may be available in the records of the Coal Authority at Mansfield, but there is otherwise little reliable information reported in the open literature. Biosolids were also used at a number of Scottish reclamation sites including Heathland Forest (Bye Law Hill) near Forth (see Section 4).