Gaining Regulatory Support For A New Bioavailable Aluminum Analysis Method

Aluminum is a toxicant to aquatic life as it can impact ion regulation and bind to fish gills causing respiratory distress. EPA promulgated aluminum aquatic life criteria to Oregon in 2021 based on their 2018 nationally recommended criteria. Aluminum is traditionally measured in environmental samples as total and dissolved aluminum, and the criteria were based on total aluminum. However, the method to measure total aluminum uses a low pH digestion which dissolves aluminum bound in sediments, such as clays and aluminosilicate minerals that are not bioavailable. This digestion method overestimates the amount of aluminum that is bioavailable and potentially toxic to aquatic life. Using total aluminum for Water Quality Assessments would assess more water bodies as impaired for aluminum toxicity than would be accurate. Waterbodies with soils high in aluminosilicates would be likely to be listed for aluminum even though there is no concern for aluminum toxicity. These listings could require excessive TMDLs to be developed for a water quality criterion that cannot be met due to the presence of sediment-bound aluminum. For NPDES permittees using alum in treatment processes to control phosphorus, using total aluminum as the regulatory metric could result in unnecessarily restrictive aluminum limits which may impact the ability of water resource recovery facilities (WRRFs) to use alum to meet phosphorus limits. Therefore, in implementation efforts, EPA recognized a new analytical method (Rodriguez et al. 2019) as potentially useful to measure bioavailable aluminum. The new method uses a less aggressive acid digestion that does not dissolve mineralized aluminum and only measures the fraction that is potentially toxic to aquatic life. Although the new analytical method is allowed for use in river measurements, it has not been codified in 40 CFR 136 as a method acceptable for NPDES permitting, and EPA encouraged further method development. Clean Water Services (CWS) is committed to studying analytical methods that best measure the potential toxicity of aluminum in the environment. The CWS Water Quality Lab began using the bioavailable aluminum analytical method in 2019 and analyzed total, bioavailable, and dissolved aluminum in concurrent effluent and river samples from the Tualatin River. This sampling effort aimed to build a larger dataset of bioavailable aluminum in natural waters and better understand aluminum fractions in local rivers. Results consistently showed that only a small fraction of the total aluminum was bioavailable and toxic to aquatic life in the Tualatin River. In paired samples collected in 2021 - 2023, upstream of the first of two WRRFs that use alum, the fraction of aluminum that was bioavailable in the river was 2-6% (n=30). In paired samples during the same time frame, downstream of both WRRFs that use alum, the fraction of aluminum that was bioavailable was up to 56%, which was expected because the effluent aluminum is nearly all bioavailable (n=30). Bioavailable aluminum appears to attenuate in the river, most likely by adsorption to clays and minerals and subsequent settling (Figure 1). The aluminum standard is dependent on a multiple linear regression model that uses three input parameters (pH, dissolved organic carbon, and total hardness) to calculate a pair of acute and chronic instantaneous water quality criteria (IWQC). Therefore, the aluminum criteria are variable and specific to the results of an individual sample. The measured aluminum concentration is compared to the calculated IWQC. To normalize the IWQC and easily compare to the appropriate concentration, toxic units are calculated for each sample by dividing the aluminum concentration by the criterion. Toxic units greater than one represent samples where aluminum concentrations exceed their criterion. In CWS ambient samples, bioavailable aluminum concentrations were always less than the IWQC. However, using the corresponding total aluminum concentrations, half of the samples yielded results greater than their corresponding IWQC. An example is given from river samples taken on May 2, 2023, that show chronic toxic units calculated from bioavailable aluminum vs total aluminum (Figure 2). Data also demonstrated that effluent aluminum from CWS WRRFs that use alum is almost 100% bioavailable. The opportunity to use bioavailable aluminum was found to substantially influence reasonable potential and calculation of effluent limits. CWS has also collaborated with Oregon DEQ to monitor total, dissolved, and bioavailable aluminum in a broader range of rivers, particularly those which would be listed for aluminum using total aluminum as the metric because they are in watersheds with soils high in clay. Split samples were analyzed by both laboratories, and side-by-side comparisons showed good reproducibility. These findings increased the confidence of both laboratories in accepting the validity of the bioavailable aluminum measurements. Further, the bioavailable fractions of aluminum from samples across the state were 2 — 16%, showing evidence that aluminum is largely bound in mineral form within clays and silicates. CWS plans to continue bioavailable aluminum monitoring efforts and collaboration with Oregon DEQ in support of this method. ASTM publication, as well as 40 CFR approval, are important next steps in wider acceptance of the method by EPA and DEQ in future aluminum criteria updates.