Unconfined Chalk Aquifer Research Articles

Concealed by darkness: Combination of NMR and HRMS reveal the molecular nature of dissolved organic matter in fractured-rock groundwater and connected surface waters.

Detailed molecular composition of solid phase extracted dissolved organic matter (SPEDOM) collected from fractured-rock groundwater was compared to connected surface river water at two different watersheds in the unconfined chalk aquifer of Champagne in France using full scan ultrahigh resolution electrospray and photoionization Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), Orbitrap tandem MS (MS/MS) and 1H magnetic resonance spectroscopy (NMR). 1H NMR spectroscopy indicated that groundwater SPEDOM carried a higher contribution of aliphatic compounds while surface river waters SPEDOM were enriched in carboxyl-rich alicyclic molecules (CRAM), acetate derivatives and oxygenated units. Furthermore, we show here that use of photoionization (APPI(+)) in aquifer studies is key, ionizing about eight times more compounds than ESI in surface river water samples, specifically targeting the dissolved organic nitrogen pool, accounting for more than 50% of the total molecular space, as well as a non-polar, more aromatic fraction; with little overlap with compounds detected by ESI(-) FT-ICR MS. On the other hand, groundwater SPEDOM samples did not show similar selectivity as less molecular diversity was observed in APPI compared to ESI. Mass-difference transformation networks (MDiNs) applied to ESI(-) and APPI(+) FT-ICR MS datasets provided an overview of the biogeochemical relationships within the aquifer, revealing chemical diversity and microbial/abiotic reactions. Finally, the combination of ESI(-) FT-ICR MS and detailed Orbitrap MS/MS analysis revealed a pool of polar, anthropogenic sulfur-containing surfactants in the groundwaters, likely originating from agricultural runoff. Overall, our study shows that in this aquifer, groundwater SPEDOM contains a significantly reduced pool of organic compounds compared to surface river waters, possibly related to a combination of lack of sunlight and adsorption of high O/C formulas to mineral surfaces.

A conceptual model of hydrogeological function and perchlorate transfer in the unconfined Champagne Chalk aquifer (NE France)

Abstract Groundwater of the unconfined Chalk aquifer in Champagne-Ardenne (NE France) is contaminated by perchlorate ( ClO 4 − ), a persistent water-soluble anion. The Chalk aquifer is a crucial water resource of the region, with complex hydraulic properties. The presence of ClO 4 − is of concern due to its potential adverse human health effects. In France, three sources of ClO 4 − contamination are suspected: industrial, military and agricultural. Both a comprehensive understanding of hydrogeological characteristics of the aquifer, and a sound knowledge of sources and behaviour of ClO 4 − in groundwater are required to allow the sustainable use of this groundwater resource. From data acquired during hydrogeological, geochemical, isotopic ( ClO 4 − ) and groundwater age (CFCs and SF 6 ) studies, and historical investigations in a study area located east of Reims, a conceptual model of Chalk aquifer function and ClO 4 − transfer has been established. High spatio-temporal heterogeneities in the unconfined Chalk aquifer are discussed. Different correlations between ClO 4 − , major ions and groundwater-level fluctuations are shown and interpreted, highlighting main factors governing the Chalk groundwater geochemistry and ClO 4 − transfer mechanisms, including water-level fluctuation, groundwater residence time, thickness of the unsaturated zone, superficial formations, distribution of fissure networks, aquifer–river relationships, origin and location of ClO 4 − in soil and human activity.

Reactive transport modelling of groundwater chemistry in a chalk aquifer at the watershed scale

This study investigates thermodynamics and kinetics of water–rock interactions in a carbonate aquifer at the watershed scale. A reactive transport model is applied to the unconfined chalk aquifer of the Champagne Mounts (France), by considering both the chalk matrix and the interconnected fracture network. Major element concentrations and main chemical parameters calculated in groundwater and their evolution along flow lines are in fair agreement with field data. A relative homogeneity of the aquifer baseline chemistry is rapidly reached in terms of pH, alkalinity and Ca concentration since calcite equilibrium is achieved over the first metres of the vadose zone. However, incongruent chalk dissolution slowly releases Ba, Mg and Sr in groundwater. Introducing dilution effect by rainwater infiltration and a local occurrence of dolomite improves the agreement between modelling and field data. The dissolution of illite and opal-CT, controlling K and SiO2 concentrations in the model, can be approximately tackled by classical kinetic rate laws, but not the incongruent chalk dissolution. An apparent kinetic rate has therefore been fitted on field data by inverse modelling: 1.5×10−5 molchalkL −1wateryear −1. Sensitivity analysis indicates that the CO2 partial pressure of the unsaturated zone is a critical parameter for modelling the baseline chemistry over the whole chalk aquifer.

Robust evidence for random fractal scaling of groundwater levels in unconfined aquifers

This study introduces new approaches to improve the statistical robustness of techniques for quantifying the fractal scaling of groundwater levels, and uses these techniques to investigate scaling of groundwater levels from a consolidated permeable carbonate aquifer. Six groundwater level time series and an associated river stage time series from the unconfined Chalk aquifer (a dual-porosity, fractured limestone aquifer) in the Pang–Lambourn catchment, UK, have been analysed. Surrogate data of time series with known scaling properties have been used to estimate the probability distribution of the spectral and geometric scaling exponents determined by detrended fluctuation analysis (DFA) and power spectral density (PSD) respectively; robust regression techniques have been used to improve estimates of the scaling exponents; and robust non-parametric techniques have been used to correlate scaling exponents with features of the boreholes and catchments. Strong statistical support has been found for temporal scaling of groundwater levels over a wide range of time scales, however, bootstrap estimates of the scaling exponents indicate a much larger range of exponents than found by previous studies, suggesting that the uncertainty in existing estimates of scaling exponents may be too small. There is robust evidence that geometrical scaling properties at each borehole can be related to the depth of the observation boreholes and distance of those boreholes from the river in the catchment, but no such correlations were found for the spectral scaling exponents. The results build on the body of evidence that groundwater levels, as with many hydrogeological phenomena, may be well modelled with mathematical concepts from statistical mechanics that do not attempt to capture every detail of these highly heterogeneous and complex systems.

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.

Transport of radionuclides in an unconfined chalk aquifer inferred from U-series disequilibria

U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for 238U, 234U, 232Th and 230Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas ( 234U/ 238U) and ( 230Th/ 232Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in 234U relative to 238U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between ( 230Th/ 232Th) activity ratios and 87Sr/ 86Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by ( 234U/ 238U) > 1 resulting from α-recoil effect of 234Th. Groundwaters are characterized by a more radiogenic signature in 87Sr/ 86Sr than the rocks. Moreover, ( 230Th/ 232Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10–35 and from 1 × 10 4 to 2 × 10 5 were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km 2), a characteristic migration time from recharge to riverine discharge of 200–600 yr for U and 0.2–3.7 Myr for Th was obtained.

Indirect emissions of nitrous oxide from regional aquifers in the United Kingdom.

Diffuse pollution of groundwater by agriculture has caused elevated concentrations of nitrate (NO3-) and nitrous oxide (N2O) in regional aquifers. N2O is an important "greenhouse" gas, yet there are few estimates of indirect emissions of N2O from regional aquifers. In this study, high concentrations of N2O (mean 602 nM) were measured in the unconfined Chalk aquifer of eastern England, in an area of intensive agriculture. In contrast, pristine groundwaters from upland regions of England and Scotland, with predominantly natural vegetation cover, were found to have much lower concentrations of N2O (mean 27 nM). A positive relationship between N2O and NO3- concentrations and delta18O-NO3 values of between 3.36 and 16.00/1000 suggest that nitrification is the principal source of N2O. A calculated emission factor (EF5-g) of 0.0019 for indirect losses of N2O from Chalk groundwater is an order of magnitude lower than the value of 0.015 currently used in the Intergovernmental Panel on Climate Change (IPCC) methodology for assessing agricultural emissions. A flux of N2O from the major UK aquifers of 0.04 kg N2O-N ha(-1) a(-1) has been calculated using two approaches and suggests that indirect losses of N2O from regional aquifers are much less significant (<1%) than direct emissions from agricultural soils.

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° 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 μg/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 μg/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.

Potential for isoproturon, atrazine and mecoprop to be degraded within a chalk aquifer system

The potential for herbicide degradation in an unconfined chalk aquifer was examined by collecting and spiking fresh samples and incubating them in the laboratory. The microcosms were incubated at 20°C under aerobic conditions and spiked with either isoproturon, atrazine or mecoprop at a concentration of 100 μg/l. The samples were obtained from a single fieldsite within the Upper Chalk aquifer in Hampshire, UK. Groundwater samples required the presence of sterile chalk in a ratio of at least 1:13 to promote isoproturon degradation. An isoproturon degradation potential existed in the soil, and the chalk unsaturated and saturated zones. However, no degradation of isoproturon in the unsaturated zone was observed when a more appropriate simulation of in-situ moisture conditions was carried out. Apart from the soil, no potential for atrazine or mecoprop degradation could be detected in the same samples over a 200-day incubation. In a series of groundwater samples taken from different boreholes, 10–300 m apart, large differences in isoproturon degradation potential were observed. Removal rates for 100 μg/l isoproturon varied from 83–425 ng/day, but in some samples no degradation potential could be detected. The primary metabolite which could be distinguished from isoproturon degradation in chalk and groundwater was monodesmethyl–isoproturon. When a chalk groundwater sample was spiked with isoproturon at 0.9 μg/l, this was not degraded over a 300-day incubation period. Further experiments with fresh groundwater from a Triassic Sandstone site illustrated that groundwater bacteria could degrade isoproturon at the more realistic temperature of 10°C as well as at 20°C.

Modelling of nitrate leaching from arable land into unsaturated soil and chalk 1. Development of a management model for applications of sewage sludge and fertilizer

A mathematical model has been produced to examine the impact of sewage sludge and fertilizer application to arable land and the effect of different crop regimes on the amount of nitrate leached to chalk groundwater. Previous work on nitrate leaching has concentrated on either a soil science or a hydrogeological approach with little overlap between the two. This study considered both fields to obtain an overall picture of the nitrate leaching process. IMPACT is a layered deterministic N-leaching model which predicts the nitrogen loads entering groundwater daily from arable land, and can be used as a management tool in development of sludge application and agricultural policy. The model relates nitrogen species movement resulting from the application of sewage sludge and fertilizer to differing vegetation-soil-hydrogeological conditions. Field data collected at three sites on the unconfined chalk aquifer of East Anglia, England over a two and a half year period was used to produce an initial conceptual model and to constrain the mathematical model during development. IMPACT simulates nitrogen and transport processes in the soil and unsaturated zone of the chalk. The nitrogen processes include: mineralisation of soil organic-N and sewage sludge organic-N, nitrification; crop uptake; volatilization; denitrification; and N inputs from fertilizers and precipitation. A mixing cell method is used to model solute transport in both the soil and chalk. Matrix flow and combined fissure-matrix flow are considered for the chalk. The model enables examination of the relationship between the arable/hydrogeological systems and the environmental implications of sludge application and of different arable regimes. Results are of use in developing strategies for arable farming and sludge application in areas sensitive to nitrate leaching. This Part 1 paper describes the model development approach. Results of associated modelling scenarios are presented separately in the associated Part 2 paper.

A preliminary assessment of nitrous oxide in Chalk groundwater in Cambridgeshire, U.K.

Groundwater samples from boreholes and springs in the unconfined Chalk aquifer of Cambridgeshire were analysed for N 2O and other N species on a monthly basis between March 1995 and February 1996. Land use in the study area is devoted to intensive arable farming supported by the application of N-based fertilisers. All groundwater samples were strongly oversaturated with N 2O, with concentrations ranging from 13 to 320 times the saturation concentration with respect to air-equilibrated water. A very good positive correlation between N 2O and NO 3 concentrations was obtained ( r 2=0.80 ), but no relationship was established between N 2O and NO 2 or NH 4 concentrations. Concentrations of N 2O and NO 3 increased continuously in the direction of groundwater flow, with molar net gain ratios of NO 3 to N 2O varying between 204 and 410. These ratios are within the range reported in previous studies of nitrification. Corresponding dissolved O 2 levels in groundwater samples were moderately undersaturated, further indicating that the main source of N 2O in Chalk groundwater in Cambridgeshire is probably nitrification. No consumption of N 2O seems to take place within the unconfined aquifer with degassing to the atmosphere apparently being the sole mechanism for N 2O removal from groundwater. An estimated N 2O flux of around 0.05 kg N 2O ha −1 a −1 from the sampled groundwater discharge points to the atmosphere was calculated for the study area. This figure is likely to be much higher, since it does not account for diffuse N 2O emissions from groundwater seepage areas or any degassing from the unconfined aquifer through the unsaturated zone. Both these processes will contribute substantially to the total aerial flux, thus suggesting that groundwater may be a significant contributor to the global N 2O budget.

The Application Of Electrical Tomography In The Study Of The Unsaturated Zone In Chalk At Three Sites In Cambridgeshire, United Kingdom

Recent developments in field acquisition of electrical tomography data and in data-inversion technology have resulted in the production of electrical images that more accurately mirror the subsurface than is possible with conventional resistivity surveys. The technique is particularly suitable for shallow site investigations where understanding complex subsurfaces in detail is important. This approach is demonstrated with surveys carried out at three sites with thin soil cover on the unconfined chalk aquifer at Cambridgeshire, United Kingdom, to examine the detailed subsurface structure of the unsaturated zone. The measurements of apparent resistivity were analyzed using recently developed software to provide images of the electrical properties of the chalk to a depth of 6 metres. The results indicate different structures at each site. Selected profiles were tested with shallow boreholes that were logged with a specially constructed resistivity probe. The borehole results confirm the significance of the recorded images.

A Lumped Parameter Model of a Chalk Aquifer‐Stream System in Hampshire, United Kingdom

ABSTRACTA field investigation into low flow augmentation using seasonal ground‐water abstraction from the unconfined Chalk aquifer in Hampshire, United Kingdom, revealed the presence of a thin, highly transmissive and storative zone in the Chalk close to the water table. A lumped parameter model of the aquifer‐stream system incorporating a variable transmissivity and storativity distribution has been developed to explain the unusual hydrogeological features of the catchment observed during the investigation.

B. J. Connorton writes

The most important hydrogeological finding to come out of the field investigations carried out during the various phases of test pumping of Stage 1 of the Thames Groundwater Scheme was the realisation that the magnitude of transmissivity (T) and storativity (S) decline significantly with depth (Owen et al. 1977). Bearing in mind that the purpose of the scheme is to abstract ground-water at times of drought, the main implication of this finding is that aquifer parameters are likely to be appreciably lower under operational conditions than under normal seasonal conditions. The author employs the Nutbrown inverse method (Nutbrown 1975) to compute the areal distribution of T and S which are then input to the groundwater model. This particular inverse technique is based on a cyclic time integration of the conventional linear groundwater flow equation, consequently, T and S values computed by this method will necessarily correspond to some average seasonal groundwater stage condition. Morel's ‘non-linear’ model is so called because the transmissivity parameter is represented as: T = K a b (1) where K a = K a ( x , y ) ... average hydraulic conductivity, b = b( x , y , h)... saturated aquifer thickness, and h = h( x , y , t) ... groundwater head K a is obtained by dividing the T value computed from the inverse method by 50 m (assumed average aquifer thickness). Storativity is a function of areal position only as computed by the inverse technique. Essentially, the author is modelling the unconfined Chalk aquifer as though it were a vertically homogenous porous medium. Equation (1)

The interpretation of tritium in the Chalk unsaturated zone

Pore-water tritium profiles from the Chalk unsaturated zone are presented and the problems and limitations in their interpretation are evaluated in detail. A new model of the physicochemical behaviour of tritium in the soil zone is proposed, which can account for the quantity and distribution of thermonuclear tritium observed at shallow depths in the unconfined Chalk aquifer. The mechanisms of solute movement through the Chalk unsaturated zone are also discussed.

Discussion on ‘A numerical model for the perdiction of long term well yield in an unconfined chalk aquifer’ by B. J. Connorton &amp; R. N. Reed

The authors thank Mr Puri for his comments on their paper. His point about the effect of anisotropy is a valid one. From our knowledge of the study area it is reasonable to assume that within the typical dry valley situation there exists (in the horizontal plane) both anisotropy (probably with the major axis coincident with the valley axis and the minor axis at right angles to it) and heterogeneity, as indicated in Fig. 2. As Mr Puri will be aware, the radial flow model can only deal with heterogeneity (fig. 7) it cannot, by definition, deal with anisotropy. Thus the basic question to be answered here is, if by some means or other the effects of anisotropy could also be taken into account by the model would the results at the pumped well be significantly different from those already obtained? I think the answer to this lies in knowing two things: (i) the ratio between the maximum and minimum transmissivity tensors (←pha ) in the field; and (ii) the critical values of ←pha which would produce a significant deviation from the isotropic case (←pha = 1) when incorporated into our vertically and laterally heterogeneous aquifer. The former would involve some quite elaborate field tests over a statistically meaningful number of pumped wells. The latter could be investigated using regional modelling techniques, although modelling non-linear changes in aquifer parameters by such methods may prove difficult. The work carried out by Vandenberg (1977) may be a useful starting point for this

Discussion on ‘A numerical model for the perdiction of long term well yield in an unconfined chalk aquifer’ by B. J. Connorton &amp; R. N. Reed

Connorton &amp; Reed's paper on the use of a radial flow model is an interesting addition to the knowledge of chalk aquifers and their hydraulic response. There are, however, a number of points that arise and need comment. The first of these is concerned with the use of a radial flow concept in aquifers that are recognised to have a marked anisotropy. The authors have dismissed this by stating that within a short distance of the well (up to 100 m) radial symmetry can be assumed. During studies of a number of pumping tests in the Middle Chalk in east Kent, which had the good fortune of having observation wells located at right angles to the pumped well, I noticed that radial symmetry of aquifer parameters almost never occurs, even at distances sis near as 50 m. The second comment relates to the use of ‘saturated thickness’ and the associated corrections for its decrease. Assigning a value related to the frequency of fissuring implies complete absence of pore water, which plays an important role in the aquifer's hydraulic response. Owen &amp; Robinson (1978) in the same study area suggest that these thicknesses range from 45 to 60 m, while the authors have implied it to be 100 m as shown by their use of adjusted T values for their modelling. The complexities involved indicate that the best way to understand the interactions between various parameters is to use a simple model, such as the radial model. One of the greatest

A numerical model for the prediction of long term well yield in an unconfined chalk aquifer

Summary The prediction of the long term yield from individual pumped wells is one of the main problems to arise from the Thames Ground Water Scheme. A numerical method for solving this problem, more versatile than conventional non-equilibrium pumping test analysis techniques, is described. The method is based on the discrete space/discrete time radial flow model, developed recently by Rushton et al. (1974). Factors modelled include the detailed spatial distribution of aquifer parameters, a variable pumping rate, natural recession and wellfield interference effects. An outline of the fundamental hydrogeological aspects of the modelling work is also given.