Daren Gooddy

Metal ion binding by natural organic matter: from the model to the field.

With the newly developed NICCA-Donnan model, we estimate the activity of toxic metal ions from simple measurements like total metal concentration and organic matter content. The model evaluates Cu and Cd binding from three field systems, a mountain lake and two sandy soils, using model parameters calibrated for natural organic matter analogues with laboratory measurements. This is possible because the model includes site binding heterogeneity, electrostatic effects, competitive binding, and ion specific nonideality. The predictions derived closely matched the field observations when site binding densities are adjusted. The partition coefficients between soil and soil solution were also predicted for Cd and Cu under conditions where the organic matter controls the metal binding in both soil and soil solution. The model calculations show that in soil solutions 50% of the Cd and 99.99% of the Cu is bound to the dissolved organic matter. The model can be used to evaluate the effects of variations in the chemical conditions (e.g., acidification or total metal loading) on the free metal ion concentration in solution.

Penetration of herbicides to groundwater in an unconfined chalk aquifer following normal soil applications

The persistence and penetration of the herbicides isoproturon and chlorotoluron in an unconfined chalk aquifer has been monitored over a 4-year period through soil sampling, shallow coring and groundwater monitoring. Chlorotoluron was applied on plots as a marker compound, having never been used previously on that, or surrounding fields. The fieldsite had a 5 degree slope with soil depths of 0.5 to 1.5 m and a water table between 20 and 5 m from the soil surface. Where the water table was deepest (9-20 m below surface (mbs)) little or no positive herbicide detections were made. However, where the water table was at only 4-5 mbs, a regular pesticide signal of around 0.1 microg/l for isoproturon and chlorotoluron could be distinguished. Over the winter recharge period automatic borehole samplers revealed a series of short-lived peaks of isoproturon and chlorotoluron reaching up to 0.8 microg/l. This is consistent with a preferential flow mechanism operating at this particular part of the field. Such peaks were occurring over 2 years after the last application of these compounds. Shallow coring failed to uncover any significant pesticide pulse moving through the deep unsaturated zone matrix at the fieldsite.

A review of the impact of climate change on future nitrate concentrations in groundwater of the UK

This paper reviews the potential impacts of climate change on nitrate concentrations in groundwater of the UK using a Source-Pathway-Receptor framework. Changes in temperature, precipitation quantity and distribution, and atmospheric carbon dioxide concentrations will affect the agricultural nitrate source term through changes in both soil processes and agricultural productivity. Non-agricultural source terms, such as urban areas and atmospheric deposition, are also expected to be affected. The implications for the rate of nitrate leaching to groundwater as a result of these changes are not yet fully understood but predictions suggest that leaching rate may increase under future climate scenarios. Climate change will affect the hydrological cycle with changes to recharge, groundwater levels and resources and flow processes. These changes will impact on concentrations of nitrate in abstracted water and other receptors, such as surface water and groundwater-fed wetlands. The implications for nitrate leaching to groundwater as a result of climate changes are not yet well enough understood to be able to make useful predictions without more site-specific data. The few studies which address the whole cycle show likely changes in nitrate leaching ranging from limited increases to a possible doubling of aquifer concentrations by 2100. These changes may be masked by nitrate reductions from improved agricultural practices, but a range of adaption measures need to be identified. Future impact may also be driven by economic responses to climate change.

Redox-driven changes in porewater chemistry in the unsaturated zone of the chalk aquifer beneath unlined cattle slurry lagoons

Farm waste stores such as cattle slurry lagoons are widespread in the UK and many overly important aquifers. Stores can be serious risks to water quality because they are important sources of N species, organic C and pathogenic microbes. At two sites on the Chalk aquifer of southern England, inclined boreholes were drilled and cored to obtain aquifer material from directly beneath unlined slurry stores. Vertical boreholes were also drilled adjacent to the slurry stores to determine any lateral movement of contaminants. Interstitial porewaters were analysed for major and minor ions and S isotopes. At the second site, unsaturated zone gases were sampled from the inclined hole. Infiltration of slurry into the unsaturated zone caused significantly elevated concentrations of metals such as Cu and Ni at both sites. Sulphate reduction was occurring at Site 1, as evidenced by SO4 concentrations decreasing from 150 to 50 mg/l and enhanced ratios of δ34S–SO4 and δ18O–SO4. Ammonium-N also leaches along with dissolved organic C which were found 17 m below ground surface at concentrations up to 400 and 260 mg/l, respectively. Contaminant concentrations were similar in the porewaters from both the inclined and vertical boreholes. At Site 2, higher contaminant concentrations were found in the inclined borehole compared with the vertical borehole. Organic C concentrations were considerably lower than at Site 1, ranging from 10 to 70 mg/l. Ammonium–N concentrations reached a maximum concentration of 25 mg/l, however NO3-N concentrations were up to 500 mg/l and SO4 concentrations were generally higher than Site 1. Data for N2/Ar and δ15N–N2 from the gas samplers show a peak of 102 and 2.2‰, respectively, at 14 m below ground level indicating denitrification was taking place. Evidence from δ34S–SO4 and δ18O–SO4 suggest that some SO4 reduction was taking place simultaneously. From CH4 and NH3 detected at depth it is suggested that slurry contamination, emanating from early use of the store, has passed through the top 18 m of the unsaturated zone at Site 2. The presence of high concentrations of NO3 and lower concentrations of organic C suggests that this lagoon has formed a relatively impermeable seal at its base within the first few years of its lifetime. The anoxic conditions at both sites may have mobilised U from N–P–K fertilisers. Both sites are continuing to impact on the porewater chemistry and pose a risk of groundwater contamination.

Understanding groundwater, surface water and hyporheic zone biogeochemical processes in a Chalk catchment using fluorescence properties of dissolved and colloidal organic matter

Understanding groundwater–surface water (GW–SW) interaction in Chalk catchments is complicated by the degree of geological heterogeneity. At this study site, in southern England (United Kingdom), alluvial deposits in the riparian zone can be considered as a patchwork of varying grades and types with an equally varied lateral connectivity. Some display good connection with the river system and others good connection with the groundwater system and by definition poorer connectivity with the surface water. By coupling tangential flow fractionation (TFF) with fluorescence analysis we were able to characterise the organic matter in the river and hyporheic zone. There is a significant proportion of particulate and colloidal fluorescent organic matter (FOM) within the river system, and at depth within the gravels beneath the river channel. At depth in the hyporheic zone the surface water inputs are dampened by mixing with deeper groundwater FOM. The shallow (0-0.5 m below river bed) hyporheic zone is highly dynamic as a result of changing surface water inputs from upstream processes. Labile C in the form of protein-like FOM appears to be attenuated preferentially compared to fulvic-like fluorescence in the hyporheic zone compared to the adjacent gravel and sand deposits. These preliminary findings have important implications for understanding nutrient and trace element mobility and attenuation within the groundwater, surface water and hyporheic zone of permeable Chalk catchments. Fluorescence analysis of dissolved organic matter has been shown to be a useful environmental tracer that can be used in conjunction with other methods to understand GW-SW processes within a permeable Chalk catchment.

Metal ion geochemistry in smelter impacted soils and soil solutions

As part of an extended project to understand the speciation of metal ions in contaminated lands, we conducted a series of chemical extraction experiments on soil and extracted the soil pore water to determine the speciation of Pb, Cd and Cu in a smelter impacted site. The chemical extractions show that soil organic matter and metal oxides (Fe and Mn) control the speciation of Pb, Cu and Cd in the soil. For Pb, these results are in agreement with previous EXAFS data on the same soils. The soil solution speciation is calculated with the help of the NICA-Donnan modelling approach. The modelling shows that Pb and Cu speciation is dominated by the dissolved organic matter while Cd is mainly in solution as a free aquo-ion. The speciation in the soil is also simulated by a model coupling both the binding to soil organic matter and metal oxides. The simulated partition coefficient (K d ) is in good agreement with the experimentally measured K d .

Derivation of lowland riparian wetland deposit architecture using geophysical image analysis and interface detection

For groundwater-surface water interactions to be understood in complex wetland settings, the architecture of the underlying deposits requires investigation at a spatial resolution sufficient to characterize significant hydraulic pathways. Discrete intrusive sampling using conventional approaches provides insufficient sample density and can be difficult to deploy on soft ground. Here a noninvasive geophysical imaging approach combining three-dimensional electrical resistivity tomography (ERT) and the novel application of gradient and isosurface-based edge detectors is considered as a means of illuminating wetland deposit architecture. The performance of three edge detectors were compared and evaluated against ground truth data, using a lowland riparian wetland demonstration site. Isosurface-based methods correlated well with intrusive data and were useful for defining the geometries of key geological interfaces (i.e., peat/gravels and gravels/Chalk). The use of gradient detectors approach was unsuccessful, indicating that the assumption that the steepest resistivity gradient coincides with the associated geological interface can be incorrect. These findings are relevant to the application of this approach in settings with a broadly layered geology with strata of contrasting resistivities. In addition, ERT revealed substantial structures in the gravels related to the depositional environment (i.e., braided fluvial system) and a complex distribution of low-permeability putty Chalk at the bedrock surface—with implications for preferential flow and variable exchange between river and groundwater systems. These results demonstrate that a combined approach using ERT and edge detectors can provide valuable information to support targeted monitoring and inform hydrological modeling of wetlands.

Behaviour and impact of cow slurry beneath a storage lagoon. II, Chemical composition of chalk porewater after 18 years

To determine the pollution hazard associated with the long-term storage of cow slurry, two boreholes were drilled to a depth of nearly 35 m in the unsaturated zone of the Upper Chalk beneath an unlined, earth-banked lagoon. Chalk porewater was extracted by centrifugation from successive 0.45 m length core sections and their chemical and biological composition determined. Porewaters from the first borehole, which was sited in the deepest part of the lagoon, were discoloured and showed the highest concentrations of bicarbonate (HCO3), dissolved organic carbon (TOC), ammonium-nitrogen (NH4-N) and organic phosphorus (Po) in the first 6 m directly beneath the base of the lagoon. Below this depth, element concentrations decreased more sharply and amounts of nitrate-nitrogen (NO3-N) increased. Porewaters from the second borehole, which was sited at the edge of the lagoon, were almost colourless and showed less elevated concentrations of determinants compared to the first borehole with the exception of NO3-N. However, large increases in TOC, NH4-N and Po were observed at 29 m in the second borehole indicating that the borehole had intercepted slurry which had migrated rapidly through the chalk profile by preferential flow along fissures in the Chalk. There was visible evidence of slurry contamination on fissure faces of chalk cores extracted from both boreholes. Microbial activity was detected only on fissure faces and not in the porewaters of either borehole. However microbially mediated reactions were important in terms of the chemical transformations (organic carbon oxidation, nitrification, nitrate reduction) taking place beneath the lagoon.

Nitrogen sources, transport and processing in peri-urban floodplains

Peri-urban floodplains are an important interface between developed land and the aquatic environment and may act as a source or sink for contaminants moving from urban areas towards surface water courses. With increasing pressure from urban development the functioning of floodplains is coming under greater scrutiny. A number of peri-urban sites have been found to be populated with legacy landfills which could potentially cause pollution of adjacent river bodies. Here, a peri-urban floodplain adjoining the city of Oxford, UK, with the River Thames has been investigated over a period of three years through repeated sampling of groundwaters from existing and specially constructed piezometers. A nearby landfill has been found to have imprinted a strong signal on the groundwater with particularly high concentrations of ammonium and generally low concentrations of nitrate and dissolved oxygen. An intensive study of nitrogen dynamics through the use of N-species chemistry, nitrogen isotopes and dissolved nitrous oxide reveals that there is little or no denitrification in the majority of the main landfill plume, and neither is the ammonium significantly retarded by sorption to the aquifer sediments. A simple model has determined the flux of total nitrogen and ammonium from the landfill, through the floodplain and into the river. Over an 8 km reach of the river, which has a number of other legacy landfills, it is estimated that 27.5 tonnes of ammonium may be delivered to the river annually. Although this is a relatively small contribution to the total river nitrogen, it may represent up to 15% of the ammonium loading at the study site and over the length of the reach could increase in-stream concentrations by nearly 40%. Catchment management plans that encompass floodplains in the peri-urban environment need to take into account the likely risk to groundwater and surface water quality that these environments pose.

Characterization of suboxic groundwater colloids using a multi-method approach

Anoxic groundwater colloid properties were measured using a minimally perturbing procedure for sampling, processing, and analysis. Analytical methods included atomic force microscopy (AFM), flow field flow fractionation (FlFFF), and transmission and scanning electron microscopy (TEM and SEM). Shallow groundwater samples showed abundant iron rich nanoparticles (NP) with diameters of 10-30 nm as well as a smaller heterogeneous polydisperse dissolved organic matter (DOM) fraction. AFM results showed NP with average heights of 10 ± 2 nm, which was corroborated by high-resolution TEM and SEM. FlFFF with UV254 nm detection found particles with number average diffusion coefficients of 2-3 × 10(-10) m(2) s(-1) and hydrodynamic diameters between 1.5 and 2 nm probably representing smaller organic macromolecules. Aeration of the samples resulted in extensive agglomeration of NP to form larger (>50 nm) colloids, and a reduction of UV-absorbing material in the 0.5-4 nm range. The complementary methods described have potential applications for investigating the fate and transport of NP in suboxic hotspots such as leachate plumes, wastewater treatment plants, and within the hyporheic mixing zone.