SWCTL

August 12, 2024

Surface and Groundwater Cleanup – Aluminum Aquatic Toxicity 

Florida has established default surface water cleanup target levels (SWCTLs), which apply to surface water and groundwater impacting a nearby surface water body. The default SWCTLs for many chemicals are lower than the groundwater cleanup target levels (GCTLs) and, in some cases, even lower than background conditions. Meeting these cleanup endpoints can be technically challenging, time-consuming, and sometimes cost-prohibitive. Today’s technical blog highlights achieving aquatic protection more sustainably and economically.

Such is the case for aluminum. The default freshwater SWCTL for aluminum is 13 µg/L, lower than the GCTL (200 µg/L) and typical background levels (31-2790 ug/L, Miami-Dade County 2022). This SWCTL, which aims to protect aquatic life from chronic toxicity, was calculated by the University of Florida in the early 2000’s (Technical Report, FDEP 2005). Fortunately, the United States Environmental Protection Agency (US EPA) has since enhanced how we evaluate aluminum toxicity to aquatic life, reflecting the latest scientific knowledge and resulting in a more practical cleanup standard.

US Environmental Protection Agency Water Quality Criteria for Aluminum Aquatic Toxicity 

In December 2018, USEPA released the Final Aquatic Life Ambient Water Quality Criteria for Aluminum 2018 (USEPA, 2018). This document outlines an approach to calculate site-specific acute and chronic freshwater aluminum thresholds. Similar to the default SWCTL, the USEPA approach considers the most sensitive relevant species and the median lethal concentration (LC₅₀) of aluminum for that species. However, the USEPA approach also accounts for variations in site-specific water chemistry, including pH, dissolved organic carbon (DOC), and total hardness, which are known to influence aluminum bioavailability, as follows:

• The pH of water affects aluminum speciation and solubility.
• The presence of DOC reduces bioavailability, as aluminum can sorb to DOC, such as humic and fulvic acids, and form organic aluminum complexes.
• Hardness affects the toxicity of aluminum, as the Al⁺³ cation competes with other cations present in water, such as calcium (Ca⁺²), for uptake. The observed effect of total hardness may be due to one or more interrelated ions, such as hydroxide, carbonate, calcium, and magnesium. Additionally, aquatic life is generally more sensitive to aluminum in soft water than hard water, although there is a reduced effect of total hardness at elevated pH levels.

Multiple linear regression (MLR) models were developed to characterize the bioavailability of aluminum in aquatic systems based on the effects of pH, DOC, and total hardness on aluminum toxicity. In addition, the agency created a user-friendly Aluminum Criteria Calculator (Aluminum Criteria Calculator V.2.0.xlsm) that allows users to enter site-specific values for pH, DOC, and total hardness to calculate the appropriate recommended freshwater acute and chronic criteria for site-specific parameters. In summary, the calculator uses the LC_50, but normalizes it using the site-specific pH, DOC, and total hardness. In addition, the USEPA 2018 document includes tables with the recommended criteria for various water chemistry conditions.

Note that the USEPA 2018 document also outlines the MLR models and the calculator’s pH, DOC, and total hardness bounds as follows:

• The pH bounds of the MLR models range from 6.0 to 8.7, but the calculator allows the user to extrapolate the pH values used to generate the MLR models. The calculator can address waters with a pH range of 5.0 – 10.5.
• The DOC bounds of the MLR models range from 0.08 mg/L to 12.3 mg/L. In this case, the calculator’s DOC is bounded at the upper limit of the empirical MLR models’ underlying DOC data, with a maximum of 12.0 mg/L DOC.
• The total hardness bounds of the MLR models range from 9.8 mg/L to 428 mg/L, but the calculator allows the user to extrapolate the total hardness values used to generate the MLR models. The calculator can address waters with a total hardness of less than 9.8 mg/L (to a minimum limit of 0.01 mg/L).

Case Study – Aluminum Aquatic Toxicity and a Residential Development Site

SCS Engineers is currently assessing a site under development for residential use. The site has an existing on-site lake, which historically has received unknown fill. Surface water sampling results confirmed aluminum concentrations (31.2 µg/L – 219 µg/L) in excess of the default SWCTL (13 µg/L). The regulatory agency has requested additional assessment and monitoring based on the sampling results and site conditions. Given the time and cost of this approach and the uncertainty in a defined path to closure, SCS instead opted to derive an alternative SWCTL (ASWCTL) using the USEPA approach.

Using the Aluminum Criteria Calculator V.2.0 (US EPA 2018) and site-specific water chemistry data from the site, SCS calculated acute (criterion maximum concentration, CMC) and chronic (criterion continuous concentration, CCC) criteria. The acute criterion protects 95% of the species in a representative aquatic community from the acute effects of aluminum. The chronic criterion represents the maximum concentration of aluminum, protecting most aquatic organisms from unacceptable short or long-term effects.

We calculated site-specific CMC and CCC values using site-specific pH, DOC, and total hardness. Site-specific pH (6.83 – 8.39) and total hardness (151 mg/L – 204 mg/L) fell within the MLR model and the calculator bounds. DOC (3.4 mg/L – 28.2 mg/L), however, did not. Although two site DOC concentrations (14 mg/L and 28 mg/L) exceeded the MLR models and calculator upper bound, we used the maximum allowable DOC concentration of 12.0 mg/L in the calculator. This approach is conservative since lower DOC concentrations result in higher bioavailability and, in turn, increased toxicity.

After evaluating the calculated CMC and CCC for each scenario, we selected the chronic criterion of 550 µg/L to serve as the ASWCTL. Using this ASWCTL, SCS recommended no further assessment or monitoring for the on-site lake. This more sustainable solution will help our client meet construction schedules, reduce costs, and expedite site closure while meeting surface mandates.

About our Authors: Meet Anabel Rodriguez Garcia and Lisa Smith

References and Resources:

Aluminum Criteria Calculator V.2.0 (USEPA 2018) https://www.epa.gov/wqc/2018-final-aquatic-life-criteria-aluminum-freshwater

Technical Report, FDEP 2005: Development of Cleanup Target Levels (CTLs) for Chapter 62-777, Florida Administrative Code. February 2005, prepared by the Center for Environmental & Human Toxicology, University of Florida.

Miami-Dade County 2022. Background Concentrations of Metals in Groundwater – Miami-Dade County. August 11, 2022. https://www.miamidade.gov/environment/library/reports/2022-08-11-background-concentrations-of-metals-in-groundwater.pdf

USEPA 2018. Final Aquatic Life Ambient Water Quality Criteria for Aluminum 2018. EPA/822/R-18/001. December 2018 https://www.epa.gov/sites/default/files/2018-12/documents/aluminum-final-national-recommended-awqc.pdf

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