Adsorption and precipitation reactions often dictate the availability of phosphorus in soil environments. Tripolyphosphate (TPP) is considered a form of slow release P fertilizer in P limited soils, however, investigations of the chemical fate of TPP in soils are limited. It has been proposed that TPP rapidly hydrolyzes in the soil solution before adsorbing or precipitating with soil surfaces, but in model systems, TPP also adsorbs rapidly onto mineral surfaces. To study the adsorption behavior of TPP in calcareous soils, a short-term (48 h) TPP spike was performed under laboratory conditions. To determine the fate of TPP under field conditions, two different liquid TPP amendments were applied to a P limited subsurface field site via an in-ground injection system. Phosphorus speciation was assessed using X-ray absorption spectroscopy, total and labile extractable P, and X-ray diffraction. Adsorption of TPP to soil mineral surfaces was rapid (< 48 h) and persisted without fully hydrolyzing to ortho-P. Linear combination fitting of XAS data indicated that the distribution of adsorbed P was highest (~ 30–40%) throughout the site after the first TPP amendment application (high water volume and low TPP concentrations). In contrast, lower water volumes with more concentrated TPP resulted in lower relative fractions of adsorbed P (15–25%), but a significant increase in total P concentrations (~ 3000 mg P kg soil) and adsorbed P (60%) directly adjacent to the injection system. This demonstrates that TPP application increases the adsorbed P fraction of calcareous soils through rapid adsorption reactions with soil mineral surfaces.

Calcium phosphate minerals are typically the solubility-limiting phase for phosphate in calcareous soils. Magnesium (Mg), despite being present in high concentrations in calcareous soils, has been largely neglected in the study of formation and stabilization of soil phosphate minerals due to the high solubility of pure Mg phosphate phases. In this study, a series of four common calcium and magnesium phosphate minerals, hydroxyapatite/bobierrite and brushite/newberyite were synthesized in the presence of widely varying Mg concentrations to examine the effects of Mg substitution upon the local bonding environment and overall structure of the precipitates. Phosphorus K-edge X-Ray absorption near edge structure (XANES) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) provide insight into the local coordination environment, whereas synchrotron powder X-Ray diffraction (SP-XRD) and transmission electron microscopy (TEM) were used for structural analysis. In acidic to neutral pH, Mg-bearing brushite phases formed over a wide range of Ca:Mg ratios. In neutral to high pH systems, a short-range order amorphous calcium phosphate (ACP) with a local structure analogous with hydroxyapatite precipitated for a wide range of Ca to Mg ratios. It can be inferred that the presence of Mg in soils leads to stabilization of metastable phases: via cation substitution in brushite and via poisoning of crystal growth propagation for hydroxyapatite.

The Giant Mine (1948–1999) generated 16 Mt of Au‐bearing mill tailings (2800 mg kg ⁻¹ ) originating from a mixture of flotation tailings (84.8 wt%), calcine residues (14.4 wt.%), and arsenic trioxide roaster waste (0.8 wt.%). A water treatment system for high As mine dewatering effluent has operated since the end of mine operations, with the intermediate storage area being the Northwest Tailings Containment Area (NW‐TCA). The tailings porewater contains elevated concentrations of dissolved As, Sb, Zn, and other metals. A multi‐year water balance supported by an examination of unsaturated and saturated flow conditions and based on isotope and geochemical analysis was conducted to understand the NW‐TCA hydrological system. Hydraulic gradients indicate persistent downward flow in the south end of NW‐TCA. Water balance calculations indicate an average of 384,000 m ³ y ⁻¹ of water from the NW‐TCA entered the groundwater system during the study period (2017–2022). Stable water isotope and water chemistry measurements indicate porewater in areas of high‐water table is influenced by mine dewatering effluent. Isotope mass balance indicates 60% of the mine dewatering effluent, which contains high concentrations of As, is sourced from water cycled through the NW‐TCA. The impact of the mine dewatering effluent is minimal in areas with a deep vadose zone, containing lower concentrations of As. Hydrological simulations indicate groundwater flow rates into the deep groundwater flow system from the NW‐TCA would be modest in the absence of the mine dewater pond because the system is near net evaporative in the absence of mine dewatering effluent.

The Giant Mine, Yellowknife, NT, Canada (1948-1999) is undergoing one of the largest remediation projects in Canadian history. Closure and long-term management of tailings containment areas (TCA) is strengthened by hydrogeochemical investigation of mechanisms controlling contaminant metal(loid) release and attenuation. The Northwest TCA (NW-TCA) at the mine (As: 2890 ± 730 mg kg⁻¹; Sb: 210 ± 71 mg kg⁻¹) stores roaster residue and flotation tailings and has been subjected to weathering for the past 25 years. The TCA also retains mine dewatering effluent (As: 22.2 ± 4.7 mg L^-1; Sb: 0.79 ± 0.11 mg L^-1) prior to treatment. This study integrates solid-phase characterization, aqueous geochemistry, and in situ gas measurements to delineate depth dependent dissolution and attenuation processes. Acid-base accounting indicates the tailings pose low or no acid-generation risk. Oxygen gas concentrations indicate sulfide oxidation extends to 4 m depth. Oxidative dissolution of sulfide minerals in the vadose zone releases As, Sb, Zn, Co, Ni, and Cu to the porewater. A portion of this As is subsequently incorporated into secondary Fe(III)-(oxyhydr)oxide phases. Below the oxidation zone, reducing conditions drive As and Sb release by reductive dissolution of Fe(III)-bearing roaster calcines. At the base of the TCA, highly reducing conditions result in partial As sequestration through sulfate reduction and secondary As sulfide precipitation. Infiltration of mine dewatering effluent has resulted in elevated groundwater As in areas influenced by the holding pond. Effective management of As and Sb requires consideration of spatial- and temporal-dependent mobilization and attenuation processes within the TCA.

The Giant Mine operated from 1948 to 1999 to recover Au from arsenopyrite-hosted ore and is undergoing one of the largest remediation projects in Canadian history. Mine-waste containing flotation tailings and As-bearing roaster-generated Fe oxides was deposited in the Northwest Tailings Containment Area (NW-TCA) from 1988 to 1999. Since 2017, a longitudinal study has been undertaken at the NW-TCA to examine hydrogeological and geochemical controls. A comprehensive conceptual model was derived from integrating 8 years of field measurements (2017-2024) through an 18-m depth profile (GM7). Tailings exhibited circumneutral pH porewater, excess neutralization capacity, and elevated dissolved As concentrations (>25 mg L^-1), predominantly in the saturated tailings. Currently, As-rich tailings porewaters at GM7 infiltrate to the base of the NW-TCA and discharge through the underlying fractured bedrock. Field measurements of the tailings hydrogeology, isotope geochemistry, mineralogy and microbiology, coupled with multicomponent reactive transport modelling, indicate that: (i) reductive dissolution of As-bearing Fe oxides controls As mobility in the NW-TCA, (ii) As released from oxidative dissolution of arsenopyrite is primarily attenuated by Fe-arsenate phases at the near-surface, and (iii) adsorption of arsenite onto existing Fe oxides is a likely As attenuation mechanism in the saturated tailings. This has profound implications for the assessment of the Giant Mine tailings, where Fe oxides represent the most abundant As-bearing phase and thus constitute a long-term source of As, driven by microbial activity and available C sources in the tailings. These findings provide direction to optimize remediation efforts of As-bearing roaster residues and sulfidic flotation tailings.