Bias in determining aluminum concentrations: Comparison of digestion methods and implications on Al management

Aluminum is an important aquatic contaminant due to its ubiquity, toxicity and low regulatory discharge limits. Aluminum is mobilized in mining related, acidic drainage and is commonly a regulated pollutant. However, while aquatic toxicity studies and toxicity criteria are based on dissolved aluminum(Ald), discharge levels are, for statutory reasons, based on total recoverable aluminum (Alt). The rationale for using total recoverable aluminum recognizes the potential for the release of exchangeable, toxic cations or dissolution of metastable metal flocs in the event the discharge enters an acidic receiving stream. The digestion methods used in determining total recoverable metals are not meant to dissolve aluminosilicate clay particles but we found that they do, resulting in positively biased total recoverable aluminum values. This study explored the interaction between total suspended solids (TSS) and total recoverable aluminum using three digestion methods to evaluate which method introduced the least bias. Using field collected water and sediment samples from two coal mine drainage sites in Central West Virginia, three total recoverable digestion methods (USEPA Method 200.7, M1; USGS In-Bottle method, M2; and a Modified In-Bottle method, M3) were used to determine total recoverable aluminum across a range of total suspended solids concentrations. Baseline simulation experiments were conducted at pH 2.5, 3.5, 4.5 and 5.5 at different total suspended solids concentrations. Results indicated that dissolved aluminum did not respond to increasing total suspended solids concentrations while determined total recoverable aluminum increased with total suspended solids, indicating varying degrees of clay dissolution and, thus bias in the total recoverable aluminum concentration. While all three digestion methods overestimated total recoverable aluminum, at the same total suspended solids concentration, total recoverable aluminum extracted by USEPA Method 200.7 (M1) was much higher than the other two digestion methods (M2 and M3). Total recoverable aluminum from different digestion methods indicated that amount of aluminosilicate clay is digested in decreasing order: USEPA 200.7 (M1) > USGS in-bottle (M2) > modified in-bottle (M3). At pH 2.5, positive bias using methods M1, M2 and M3 was 153-287%, 53% and 40% respectively. Positive bias was greatest at pH greater than 4.5. Methods M1, M2 and M3 yielded positive biases of 660-1060%, 120-360% and 200-320% respectively. The results suggest that USEPA method 200.7 (M1) resulted in the greatest bias. Given its application in determining regulatory compliance, this is an important issue requiring further study.