Hotspots of Legacy Phosphorus in Agricultural Landscapes: Revisiting Water-Extractable Phosphorus Pools in Soils

Controlling phosphorus (P) losses from intensive agricultural areas to water bodies is an ongoing challenge. A critical component of mitigating P losses lies in accurately predicting dissolved P loss from soils, which often includes estimating the amount of soluble P extracted with a laboratory-based extraction, i.e., water-extractable P (WEP). A standard extraction method to determine the WEP pool in soils is critical to accurately quantify and assess the risk of P loss from soils to receiving waters. We hypothesized that narrower soil-to-water ratios (1:10 or 1:20) used in current methods underestimate the pool of WEP in high or legacy P soils due to the equilibrium constraints that limit the further release of P from the solid-to-solution phase. To investigate P release and develop a more exhaustive and robust method for measuring WEP, soils from eight legacy P fields (Mehlich 3–P of 502 to 1127 mg kg−1; total P of 692 to 2235 mg kg−1) were used for WEP extractions by varying soil-to-water ratios from 1:10 to 1:100 (weight:volume) and in eight sequential extractions (equivalent to 1:800 soil-to-water ratio). Extracts were analyzed for total (WEPt) and inorganic (WEPi) pools, and organic (WEPo) pool was calculated. As the ratios widened, mean WEPi increased from 23.7 mg kg−1 (at 1:10) to 58.5 mg kg−1 (at 1:100). Further, WEPi became the dominant form, encompassing 92.9% of WEPt at 1:100 in comparison to 79.0% of WEPt at 1:10. Four of the eight selected soils were extracted using a 1:100 ratio in eight sequential extractions to fully exhaust WEP, which removed a cumulative WEPt of 125 to 549 mg kg−1, equivalent to 276–416% increase from the first 1:100 extraction. Although WEP concentrations significantly declined after the first sequential extraction, WEP was not exhausted during the subsequent extractions, indicating a sizeable pool of soluble P in legacy P soils. We conclude that (i) legacy P soils are long-term sources of soluble P in agricultural landscapes and (ii) the use of a 1:100 soil-to-water ratio can improve quantification and risk assessment of WEP loss in legacy P soils.

Use of Sequential Extraction and Mercury Stable Isotope Analysis to Assess Remobilization of Sediment-Bound Legacy Mercury

The goal of this project was to assess how anthropogenic legacy mercury (Hg) retained in streambed sediment may be remobilized to stream water. To do this, we performed sequential extractions...

Soil Lead Concentration and Speciation in Community Farms of Newark, New Jersey, USA

Farmed urban soils often bear legacies of historic contamination from anthropogenic and industrial sources. Soils from seven community farms in Newark, New Jersey (NJ), USA, were analyzed to determine the concentration and speciation of lead (Pb) depending on garden location and cultivation status. Samples were evaluated using single-step 1 M nitric acid (HNO3) and Tessier sequential extractions in combination with X-ray absorption fine structure spectroscopy (XAFS) analysis. Single-step extractable Pb concentration ranged from 22 to 830 mg kg−1, with 21% of samples reporting concentrations of Pb > 400 mg kg−1, which is the NJ Department of Environmental Protection (NJDEP) limit for residential soils. Sequential extractions indicated lowest Pb concentrations in the exchangeable fraction (0–211 mg kg−1), with highest concentrations (0–3002 mg kg−1) in the oxidizable and reducible fractions. For samples with Pb > 400 mg kg−1, Pb distribution was mostly uniform in particle size fractions of <0.125–1 mm, with slightly higher Pb concentrations in the <0.125 mm fraction. XAFS analysis confirmed that Pb was predominantly associated with pyromorphite, iron–manganese oxides and organic matter. Overall results showed that lowest concentrations of Pb are detected in raised beds, whereas uncultivated native soil and parking lot samples had highest values of Pb. As most of the Pb is associated with reducible and oxidizable soil fractions, there is a lower risk of mobility and bioavailability. However, Pb exposure through ingestion and inhalation pathways is still of concern when directly handling the soil. With increasing interest in urban farming in cities across the USA, this study highlights the need for awareness of soil contaminants and the utility of coupled macroscopic and molecular-scale geochemical techniques to understand the distribution and speciation of soil Pb.

Chemical and thermal characteristics of soluble polysaccharides from fruit pericarps of the Algerian Argania spinosa

Soluble polysaccharides were isolated from fruit pericarps of the Algerian Argania spinosa. The cell wall fraction was subjected to sequential extractions with H2O (2 × 2 h at 100°C), EDTA (1%, 6 h at 80°C) and KOH (1 and 4 M, 14 h at 25°C). The structures of the obtained polysaccharide fractions were characterized using gas chromatography (GC), infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). The presence of arabinose, galactose and rhamnose in the pectin fractions suggests the presence of rhamnogalacturonan, while the abundance of xylose in the hemicellulosic fractions indicates the presence of xylan. The DSC data revealed the endothermal behavior of all the soluble polysaccharides and only two thermal transitions, the glass transition (Tg) and the fusion transition (Tf) have been recorded.

Potential Release of Zinc and Cadmium From Mine-Affected Soils Under Flooding, a Mesocosm Study

Metal-contaminated mining soils pose serious environmental and health risks if not properly managed, especially in mountainous areas, which are more susceptible to perturbation. Currently, climate change is leading to more frequent and intense rain events, which cause flooding episodes, thereby altering soil redox equilibria and contaminants stability. We evaluated the potential release of Zn and Cd (two of the most common inorganic contaminants) and the factors regulating their solubility and speciation in two heavily contaminated soils representative of a Zn-mining area. The soils were flooded under aerobic (for 24 h) and anaerobic (for 62 days) conditions using mesocosm experiments, sequential extractions, and geochemical modelling. Leaching trials under aerobic conditions showed a high release of Zn and Cd (10 times the legislative limits), with metals possibly migrating via water infiltration or runoff. Under anaerobic conditions Zn and Cd were initially released. Then, solution concentrations decreased gradually (Zn) or sharply (Cd) until the end of the experiment. Sequential extractions and multisurface modelling indicated that both metals precipitated mainly as carbonates. This was confirmed by a geochemical multisurface modelling, which also predicted the formation of sulphides after 60 days in one soil. The model calculated metals to be preferentially complexed by organic matter and well predicted the observed soil solution concentrations. The results showed that during flooding episodes contaminants could be promptly transferred to other environmental compartments. The use of multisurface modelling coupled with laboratory experiments provided useful indications on the potential release and speciation in case of anoxic conditions.

Redox Dependent Arsenic Occurrence and Partitioning in an Industrial Coastal Aquifer: Evidence from High Spatial Resolution Characterization of Groundwater and Sediments

Superlative levels of arsenic (As) in groundwater and sediment often result from industrial pollution, as is the case for a coastal aquifer in Southern Italy, with a fertilizer plant atop. Understanding conditions under which As is mobilized from the sediments, the source of that As, is necessary for developing effective remediation plans. Here, we examine hydrogeological and geochemical factors that affect groundwater As concentrations in a contaminated coastal aquifer. Groundwater has been subject to pump-and-treat at a massive scale for more than 15 years and is still ongoing. Nevertheless, As concentrations (0.01 to 100 mg/L) that are four orders of magnitude more than Italian drinking water standard of 10 μg/L are still present in groundwater collected from about 50 monitoring wells over three years (2011, 2016, and 2018). As was quantified in three different locations by sequential extractions of 29 sediment cores in 2018 (depth 2.5 m to −16.5 m b.g.l.), combined with groundwater As composition, the aqueous and solid partitioning of As were evaluated by partition coefficient (Kd) in order to infer the evolution of the contaminant plumes. Most sediment As is found in easily extractable and/or adsorbed on amorphous iron oxides/hydroxides fractions based on sequential extractions. The study shows that As contamination persists, even after many years of active remediation due to the partitioning to sediment solids. This implies that the choice of remediation techniques requires an improved understanding of the biogeochemical As-cycling and high spatial resolution characterization of both aqueous and solid phases for sites of interest.

The Role of Barite in the Post-Mining Stabilization of Radium-226: A Modeling Contribution for Sequential Extractions

Barite is ubiquitous and known to incorporate 226Ra through the formation of a solid-solution. In U mining mill tailings, barite is one of the dominant sulfate-binding minerals. In such environments, sequential extractions are generally used to identify the U- and 226Ra-binding phases and their associated reactivity. To better decipher the main processes governing the behavior of 226Ra during such sequential extractions, a geochemical model was developed with PHREEQC mimicking the sequential extraction of U and 226Ra from Bois-Noirs Limouzat U mine tailings, France. The model results were compared with a dataset produced by an experimental sequential extraction from the same mine tailings and including data on the solids and selective extraction results with the major elements, U and 226Ra. The simulations reproduced the results of the experimental chemical extractions accurately, with iron oxyhydroxides being the major U binding phase. However, the modeling indicated rather that barite would be the main 226Ra binding phase, instead of the iron oxyhydroxides identified by the experimental extractions. This is consistent with the 226Ra concentration measured in pore water, but in disagreement with the direct interpretation of the sequential extractions. The direct interpretation disregarded the role of barite in the geochemical behavior of 226Ra because barite was not specifically targeted by any of the extraction steps. However, the modeling showed that the dissolution of 226Ra-binding barite by reactants would lead to a 226Ra redistribution among the clay minerals, resulting in a skew in the experimental results. Similar results were achieved by referring simply to the bulk mineralogy of the tailings. This study highlights the importance of considering the mineralogy, mineral reactivity and retention capacity for more realistic interpretation of sequential extractions. Moreover, this paper provides new perspectives on the long-term consequences of these mill tailings in which barite controls the geochemical behavior of the 226Ra.