Except in some extremely arid environments or at higher elevations, open pit mining tends to result in the development of semi-permanent or permanent water bodies post closure, when dewatering has ceased and pre-mining water tables have recovered. Historically, little attention has been given to the development and quality of post-closure pit lakes, many of which are dotted around the landscape in mining districts.
More recently, however, regulatory interest has been focused not only on some of these existing pit lakes, but on the predicted occurrence of post-closure open pit mine lakes that are in the environmental impact phase of project permitting.
Through baseline groundwater monitoring coupled with numerical flow modelling, the hydrogeologist attempts to predict the extent of the pit lake, while the geochemist attempts to predict the chemistry of the water through the post-closure decades. But what does it all really mean? Will that pit lake end up being a long-term liability to the mining company, and how does one assess those risks?
Aside from examining the potential impacts that a pit lake may have on local groundwater resources (flow-through or not flow-through), one generally needs to assess whether the pit water will have a toxicological effect on human health, as well as terrestrial and/or volant wildlife. This is more frequently accomplished using an Ecological Risk Assessment.
The U.S. Environmental Protection Agency recognises several methods for estimating risk in their Guidelines for Ecological Risk Assessment, including: toxicity test results, assessments of existing impacts at a site, or risk calculations comparing exposures estimated for the site with toxicity values from literature. The last approach, more commonly known as the Hazard Quotient Method, calculates toxicity reference values (TRVs) using the body weights, water ingestion rates, and published toxicological data for specific ecological species. A conservative assessment will utilise No-Observed-Adverse-Effect-Level (NOAEL) studies from the toxicological literature, which represent the level of exposure which does not cause observable harm or effects. In some cases, however, a risk manager may defer to Lowest-Observed-Adverse-Effect-Level (LOAEL) data when studies are limited or inconclusive, or when the NOAEL is deemed overly conservative. LOAELs represent the lowest dose in a toxicity study resulting in adverse health effects, and, when used, can provide a more reasonable TRV calculation.
By dividing measured or predicted water quality constituent concentrations (known as exposure point concentrations) by this TRV, Hazard Quotient ratios are calculated. When a ratio is less than 1.0, there is reasonable certainty that the chemical constituent is not likely to harm the ecological receptor; when the ratio equals 1.0, the chemical constituent alone is not likely to cause ecological harm, but could have an additive effect if other constituents are also somewhat elevated; and when the ratio is greater than 1.0, the potential for adverse effects cannot be ruled out. In the end, it is the risk managers who interpret the validity, veracity, and appropriateness of these calculations when assessing the potential liability associated with the post-closure pit lake.
