Investigation into the impact of chromium contamination in the soils and groundwater underlying a manufacturing plant on a coastal aquifer
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The study area is located within the residential, commercial and industrial area, approximately 20km to the south west of the Durban CBD, between a turf club site and the international airport of Durban. Between 1945 and 1990, the site was used for the production of sodium dichromate (SDC), chromium tanning salts, chromic acid and sodium sulphate. In 1991, the production of sodium dichromate (SDC) was discontinued on the site, and manufacturing activities were limited to the production of chromium tanning salts. These salts are used in the production of leather where they are essential in converting perishable raw hides into durable leather. In 2004, an investigation was initiated in the study area following the discovery of he xava lent chromium [Cr(VI)] in groundwater. Cr(VI) was detected in groundwater samples taken from an open pit excavated j ust outside the perimeter of the manufacturing plant site. It is considered that the actual main source of the groundwater plume are suspected hot spots in the soil within aquifer 1 and 2. It is most likely that the hot spots originated from SDC spills during former production and handling at certain locations within the manufacturing plant site. It is reasonable to assume that the SDC entered the groundwater from these production and handling locations and is still present in the soil voids within aquifer 1 and 2. SDC liquid slowly dissolved the groundwater flowing around the hot spots and would appear to be feeding the observed groundwater plume at present. The specific aims of this research were to: o Provide a literature overview of chromium contamination in the subsurface o Establish the nature of geology and geohydrology underlying the manufacturing plant o Quantify the levels and extent of chromium contamination in the soils and groundwater underlying the manufacturing plant • Identify the source of chromium contamination in the soils and groundwater underlying the manufacturing plant and related potential pathways and exposure scenarios to the point of exposure of the receptors • Conduct a risk assessment for the soils and groundwater Field activities associated with this investigation included the following: • Hydrocensus survey o Installation of new boreholes • Borehole pumping tests Groundwater level monitoring o Groundwater sampling o Soil sampling at test pits A hydrocensus survey conducted within a I km radius of the plant site revealed that there were no private boreholes in or close to the affected area. The boreholes found were mainly industrial boreholes in other industries around the manufacturing plant including the turf club site. These boreholes were in the uncontaminated aquifer and most of them were either blocked or destroyed. The investigations revealed that the fill underlying the site occurs from the surface to depths in the range of approximately 0.4 metres to 2.1 metres below existing ground level. The fill generally comprises brown to dark grey, silty sand to slightly clayey sand, and contains abundant gravel and rubble in places. The fill overlies the harbour bed sediments, which generally occur in four predominantly sandy aquifer horizons interlayered with clay layers of various composition and thickness. The harbour bed sediments overlie sandstone of the Natal Group or sandy siltstones of the St Lucia Formation at depths of between approximately 28 and 32 metres below existing ground level on the manufacturing plant site. The weathered sandstone immediately below the harbour beds generally comprises residual, highly weathered, orange brown, slightly clayey to silty sand. With depth the sandstone typically becomes less weathered, grading into pinkish maroon sandstone bedrock which extends to depths in excess of 100 metres below the site. The hydraulic conductivity values of between 0.02 mid to 2.23 mid were estimated in various aquifers underlying the manufacturing plant site. The depth to the groundwater table ranged from 0.0 m to 3.1 m across the manufacturing plant site area, as measured in the installed monitoring boreholes. The elevation of the groundwater table ranged from 13.5 mMSL to 17.5 mMSL, with an inferred direction of groundwater flow towards the east in aquifers I to 3.Within aquifer 4 and the Natal formation the groundwater flow was towards the south east in principle corresponding to the general regional groundwater flow at depth from the hills towards the sea. The highest measured Cr(VI) concentrations in groundwater samples were found in aquifer I and aquifer 2 underlying closed or dismantled production facilities on the manufacturing plant site where sodium dichromate (SDC) liquid was produced or handled between 1945 and 1990. The highest measured Cr(Vl) concentrations in soil samples taken at the manufacturing plant site coincide with the above mentioned locations. Based on the site investigations, a risk assessment for the soils and groundwater underlying the study area was conducted using the RBCA approach in order to evaluate and assess the exposure scenarios. The risk assessment focused on the following exposure pathways: o Soil to human - The potential exposure of humans by ingestion, dermal contact or inhalation of Cr(VI) or Crï lll) of contaminated soil. o Soil to groundwater - The receptor or subject of protection is the groundwater with the point of exposure at the ground water surface. o Soil to plant - Concerns the potential uptake of Cr(VI) by the plants from contaminated soil/groundwater. • Groundwater - Is the migration of the Cr(VI) contamination within the ground water to any receptor. It is addressed in this context as groundwater plume or plume only. The measured concentrations both for Cr(III) and Cr(VI) in the soil samples taken on the manufacturing plant site were always below the soil screening levels (SSL's) for ingestion and dermal contact for commercial/industrial areas. Beneath certain areas of the plant site, the Cr(VI) concentrations in the soil exceeded the SSL's for inhalation of fugitive particulates. These contaminant values do not pose a health risk to workers on the plant site or on neighbouring industrial sites, as in all instances the ground surface is covered by buildings and/or paved in concrete/asphalt. The measured concentrations ofCr (VI) and Cr(IlI) in the soil samples were well below the SSL's for ingestion and dermal contact in the neighbouring area. Hence neither of the concentrations ofCr(VI) and Cr(III) found in the soils of the neighbouring area pose risk to humans. Based on the results of the risk assessment for the exposure scenario soil to groundwater, it is evident that on the manufacturing plant site outside the groundwater plume area, the Cr(VI) concentrations in the soils were below the screening levels. In the vicinity of the 'hot spots' (active sources) the Cr(VI) concentrations were above the screening levels. Therefore these contaminated soil areas have an impact on the groundwater plume. In the residential area and turf club site, the measured Cr(VI) concentrations in the soil samples outside the plume area and within the plume were all below the screening levels. Hence the migration ofCr(VI) from the soil to the groundwater in the neighbouring area is of no concern and does not pose a risk. Numerous studies and scientific papers have indicated that the soluble Cr(VI) is not taken up easily by plants. If taken up by plants or in general by living tissue it is rapidly converted to Cr(III). Cr(III) in plants does not pose any risk to human health since it is an important component of a balanced human diet. Hence the exposure scenario soil to plant to human does not pose a risk. The measured concentrations for Cr(VI) in the groundwater samples taken in the plume area exceeded all risk based screening levels for drinking water, irrigation and livestock, the contaminated groundwater is clearly not suitable for drinking, irrigation and livestock, as exposure to large quantities of the contamination could lead to serious health effects. The contaminated ground water starts approximately I to 2 meters below the ground surface, provided a person does not come into direct contact with the contaminated groundwater through drinking or skin contact, there would be no risk of adverse health effects to the person. Remediation of soil and ground water contamination at the manufacturing plant site is not expected to be a simple matter that is likely to be achieved over a short period. Therefore, it has been important to establish the risks that have to be dealt with, and to set targets for remediation that will be realistic to achieve over time. In response to regulatory obligations, the risk assessment has been used as a basis to set short-term, medium-term, and long-term targets for cleanup. The assessment has also set preliminary remediation target concentrations for chromium contamination in the soils and groundwater on the site.