Leaching of Metals from Aquifer Soils during Infiltration of Low-Ionic-Strength Reclaimed Water: Determination of Kinetics and Potential Mitigation Strategies
Year Released: 2009
Funding Partner: Bureau of Reclamation
Total Investment: $117,178.18 (Cash)
Principal Investigator: Qilin Li, Ph.D., Rice University
Reclaimed water recharged into aquifers represents a significant source of potable water. Among the high-priority research initiatives surrounding this source of water is the association of metals of public health concern with soil particles during aquifer transportation and storage. These metals are influenced by the aqueous milieu, including ionic strength, pH, and redox potential of the surrounding groundwater.
Observations from aquifer storage and recovery (ASR) sites have indicated the potential for metal mobilization in response to shifts in introduced water chemistry. Introduction of reclaimed water with TDS levels significantly lower than those in groundwater may significantly disturb chemical equilibria, possibly resulting in dissociation of some of these metals and subsequent mobilization into the groundwater. Depending on the kinetics of desorption, leached metals may produce regions of unacceptable water quality.
Goals and Objectives
The project evaluates the potential for release of metals of public health concern from surface infiltration operations when reclaimed water of low total dissolved solids (TDS) is used. The team aimed to evaluate the effects of reclaimed water chemistry and soil characteristics on trace metal immobilization, to obtain trace metal desorption kinetic data for batch and continuous-flow systems, and to develop metal transport models for evaluation of contamination mitigation strategies.
Specific goals of the project included the following:
- Evaluate the effects of solution chemistry, for example, ionic strength, pH, and redox potential, on trace metal immobilization;
- Compare the potentials for metal leaching of different soil types and identify key soil characteristics related to metal leaching;
- Obtain trace metal desorption kinetics data for batch and continuous-flow systems; and
- Develop metal transport models for evaluation of contamination mitigation strategies.
Findings and Conclusions
- Arsenic desorption increases slightly with pH because of the higher solubility of arsenic species at higher pH; this effect is, however, largely negated by the lower solubility of carbonate minerals, the main source of desorbable arsenic in the soil samples tested. The buffering capacity of dissolved carbonates stabilizes pH.
- TDS of the recharge water strongly impacts the ion exchangeable metals, with metal desorption increasing with TDS; arsenic desorption does not change significantly with TDS due to the non-detectable amount of ion exchangeable arsenic in the soil samples.
- Ionic composition of the recharge water has great impact on metal desorption. With the soil samples tested, arsenic desorption decreases with Ca concentration in the recharge water because of the reduced calcite dissolution, suggesting that lime or soda ash conditioning may be an effective strategy to mitigate arsenic leaching during the recharge.
- Desorption of all metals is biphasic, with a fast and a slow desorption site, and can be well described by a two first-order reaction model.
- Both column experiments and groundwater transport model simulation demonstrate that mixing the RO water with a recharge water of higher Ca concentration and alkalinity can effectively mitigate the metal leaching problem.
Ultimately, this report informs recharge utilities of major water quality parameters and soil characteristics that contribute to leaching of metals of public health concern and the potential risk of using RO treated wastewater in surface infiltration operations. For soil or aquifer materials whose arsenic is mainly associated with carbonate minerals, lime or soda ash conditioning or blending with a recharge water of higher Ca concentration and alkalinity is recommended for mitigating arsenic leaching.