Simon A. Mathias

Nitrate pollution in intensively farmed regions: What are the prospects for sustaining high‐quality groundwater?

[1] Widespread pollution of groundwater by nutrients due to 20th century agricultural intensification has been of major concern in the developed world for several decades. This paper considers the River Thames catchment (UK), where water-quality monitoring at Hampton (just upstream of London) has produced continuous records for nitrate for the last 140 years, the longest continuous record of water chemistry anywhere in the world. For the same period, data are available to characterize changes in both land use and land management at an annual scale. A modeling approach is used that combines two elements: an estimate of nitrate available for leaching due to land use and land management; and, an algorithm to route this leachable nitrate through to surface or groundwaters. Prior to agricultural intensification at the start of World War II, annual average inputs were around 50 kg ha−1, and river concentrations were stable at 1 to 2 mg l−1, suggesting in-stream denitrification capable of removing 35 (±15) kt N yr−1. Postintensification data suggest an accumulation of 100 (±40) kt N yr−1 in the catchment, most of which is stored in the aquifer. This build up of reactive N species within the catchments means that restoration of surface nitrate concentrations typical of the preintensification period would require massive basin-wide changes in land use and management that would compromise food security and take decades to be effective. Policy solutions need to embrace long-term management strategies as an urgent priority.

Probabilistic longevity estimate for the LUSI mud volcano, East Java

Abstract: A new method for estimating the duration of a mud volcano eruption is applied to the LUSI mud volcano in East Java. The estimate is based upon carbonates at depths in the range 2500–3500 m being the water source, with an estimated area of 100–600 km2, thickness of 0.2–1.0 km, porosity of 0.15–0.25, an initial pressure between 13.9 and 17.6 MPa, and a separate, shallower source of mud (c. 1200–1800 m depth). The resulting 50 percentile for the time it takes for flow to decline to <0.1 Ml day−1 is 26 years. By analogy with natural mud volcanoes it can be expected to continue to flow at lower rates for thousands of years. Assuming subsidence rates of between 1 and 5 cm day−1, land surface subsidence of between c. 95 and c. 475 m can be expected to develop within the 26 year time period.

Farming for Water Quality: Balancing Food Security and Nitrate Pollution in UK River Basins

Widespread pollution of groundwater by nutrients is an externality of modern intensive agriculture. Rising nitrate concentrations in freshwater have been of concern throughout the developed world for several decades. Initial worries focused on human health but more recently nitrates role in eutrophication has also become a cause for concern. Because the impact on water quality often comes decades after land use change, the challenge for science is to produce an integrated model of catchment hydrology and quality applicable to the long time-scales involved and that can cope with the complexity of connectivity among land, aquifer, and river. This article discusses the balance between food production, and therefore food security, and protection of water resources. We use recent results from a catchment-scale model of the River Thames in the United Kingdom to demonstrate that the response time of catchments can be on the order of decades, given the delays induced by groundwater flow through aquifers. Historically, the main drivers for changes in N fluxes were massive land use change associated with wartime plowing of permanent pastures and postwar modernization and intensification of agriculture, leading to the current quasi-steady state of N-dependent but leaky agriculture. It is clear that restoration of water quality to mid-twentieth-century levels would require very severe changes in land use and land management, significantly affecting UK food supply and security. Moreover, the potential timescales for recovery are well beyond those of normal political cycles. Failure to act will mean a continued high level of nitrogen transfer to rivers, estuaries, and oceans, with potentially serious ecological implications, and continued emissions greenhouse gases to the atmosphere. Notwithstanding improved efficiency of agronomic methods, the situation is unlikely to change significantly without radical shifts in legislation or farm economics.

Analytical Model for CO2 Injection into Brine Aquifers-Containing Residual CH4

During CO2 injection into brine aquifers-containing residual and/or dissolved CH4, three distinct regions develop: (1) a single-phase, dry-out region around the well-bore filled with pure supercritical CO2; (2) a two-phase, two-component system containing CO2 and brine; and (3) a two-phase, two-component system containing CH4, and brine. This article extends an existing analytical solution, for pressure buildup during CO2 injection into brine aquifers, by incorporating dissolved and/or residual CH4. In this way, the solution additionally accounts for partial miscibility of the CO2–CH4–brine system and the relative permeability hysteresis associated with historic imbibition of brine and current drainage due to CO2 injection and CH4 bank development. Comparison of the analytical solution results with commercial simulator, CMG-GEM, shows excellent agreement among a range of different scenarios. The presence of residual CH4 in a brine aquifer summons two competing phenomena, (1) reduction in relative permeability (phase interference), which increases pressure buildup by reducing total mobility, and (2) increase in bulk compressibility which decreases pressure buildup of the system. If initial CH4 is dissolved (no free CH4), these effects are not as important as they are in the residual gas scenario. Relative permeability hysteresis increased the CH4 bank length (compared to non-hysteretic relative permeability), which led to further reduction in pressure buildup. The nature of relative permeability functions controls whether residual CH4 is beneficial or disadvantageous to CO2 storage capacity and injectivity in a candid brine aquifer.

Recovering tracer test input functions from fluid electrical conductivity logging in fractured porous rocks

A radially convergent tracer test was carried out in an unconfined Chalk aquifer of Berkshire, United Kingdom. Fluorescent tracers were injected into two boreholes lying 32 m (PL10A) and 54 m (PL10B) from the abstraction hole. The tracers were also mixed with an NaCl solution so that vertical distributions of tracer within the injection wells could be monitored using fluid electrical conductivity (FEC) logging. The breakthrough curve (BTC) from PL10A was unimodal and had a first arrival time of 14 min. The BTC from PL10B exhibited two distinct peaks and a first arrival time of just 4 min. The tracer test input functions were derived by numerically modeling the observed FEC logs of the injection wells. These were then convoluted with a conventional, Fickian matrix diffusion dual-porosity model. The results suggested that the multiple peaks were due to the way in which the tracers left the injection wells and migrated into the aquifer. FEC log inversion proved to be an effective method for predicting borehole flow data obtained by flowmeters and recovering tracer test input functions for radially convergent tracer tests.

Recent advances in modelling nitrate transport in the Chalk unsaturated zone

The Chalk unsaturated zone is crucial in controlling the delivery of nitrate to Chalk streams and groundwater abstraction wells. In this paper, results from a dual-permeability numerical model of the Chalk unsaturated zone are used to illustrate the relative roles of matrix and fracture flow. A major challenge arises in representing the Chalk unsaturated zone within catchment-scale models for nutrient management. These have generally been based on simple conceptual reservoirs or compartments to represent soils and groundwater. A more appropriate conceptualization has recently been developed and applied to the Lambourn catchment within a catchment-scale nutrient model (INCA-Chalk). Preliminary results from this work are discussed, which clearly illustrate the decadal time scales that need to be considered in the context of nutrient management and the Water Framework Directive.

Multiple Well Systems with Non-Darcy Flow

Optimization of groundwater and other subsurface resources requires analysis of multiple-well systems. The usual modeling approach is to apply a linear flow equation (e.g., Darcys law in confined aquifers). In such conditions, the composite response of a system of wells can be determined by summating responses of the individual wells (the principle of superposition). However, if the flow velocity increases, the nonlinear losses become important in the near-well region and the principle of superposition is no longer valid. This article presents an alternative method for applying analytical solutions of non-Darcy flow for a single- to multiple-well systems. The method focuses on the response of the central injection well located in an array of equally spaced wells, as it is the well that exhibits the highest pressure change within the system. This critical well can be represented as a single well situated in the center of a closed square domain, the width of which is equal to the well spacing. It is hypothesized that a single well situated in a circular region of the equivalent plan area adequately represents such a system. A test case is presented and compared with a finite-difference solution for the original problem, assuming that the flow is governed by the nonlinear Forchheimer equation.

A pseudospectral approach to the McWhorter and Sunada equation for two-phase flow in porous media with capillary pressure

Two well-known mathematical solutions for two-phase flow in porous media are the Buckley–Leverett equation and the McWhorter and Sunada equation (MSE). The former ignores capillary pressure and can be solved analytically. The latter has traditionally been formulated as an iterative integral solution, which suffers from convergence problems as the injection saturation approaches unity. Here, an alternative approach is presented that solves the MSE using a pseudospectral Chebyshev differentiation matrix. The resulting pseudospectral solution is compared to results obtained from the original integral implementation and the Buckley–Leverett limit, when the capillary pressure becomes negligible. A self-contained MATLAB code to implement the new solution is provided within the manuscript. The new approach offers a robust and accurate method for verification of numerical codes solving two-phase flow with capillary pressure.

Analysis of Momentum Transfer in a Lid-Driven Cavity Containing a Brinkman-Forchheimer Medium

The CE/SE (the space-time conservation element and solution element method) scheme with the second-order accuracy has been proposed. And the pretreatment method has been introduced to convert the parabolic equations to the hyperbolic equations, which are accurately solved by the CE/SE method. The lid-driven rectangular cavity containing a porous Brinkman–Forchheimer medium is studied in this numerical investigation. The Brinkman–Forchheimer equation is used such that both the inertial and viscous effects are incorporated. The governing equations are solved by the improved CE/SE approach. The characteristics of the flow are analyzed with emphasis on the influence of the Darcy number and the cavity depth. It is found that the porous medium effect decreases both the strength and the number of eddies, especially for deep cavities.

A parameter sensitivity analysis of two Chalk tracer tests

Abstract As with most fractured rock formations, Chalk is highly heterogeneous. Therefore, meaningful estimates of model parameters must be obtained at a scale comparable with the process of concern. These are frequently obtained by calibrating an appropriate model to observed concentration–time data from radially convergent tracer tests. Arguably, an appropriate model should consider radially convergent dispersion and Fickian matrix diffusion. Such a model requires the estimation of at least four parameters. A question arises as to whether this level of model complexity is supported by the information contained within the calibration data. A four-parameter model was developed for the analysis of two Chalk aquifer radially convergent tracer tests. The parameters included an advective travel time, ta, a characteristic fracture diffusion time, tcf, a characteristic matrix block diffusion time, tcb and a Peclet number, P. Because the tracer test duration was less than 500 h, tcb was impossible to identify. Further analysis showed that a large set of correlated values of P, ta and tcf would lead to equally good model fits. To resolve this ambiguity, more and better quality data are needed at the very start of the breakthrough curve, to constrain the mechanical dispersion parameter, P.