Unconfined Zone Research Articles

Isotopic and noble gas study of Chalk groundwater in the London Basin, England

The chemical and isotopic composition of groundwater from 52 sites in the London (U.K.) area was determined as part of a project aimed at assessing the spatial variation in the age of Chalk groundwater, and in determining the relationship between fracture and matrix groundwater in this dual porosity system. Systematic changes in groundwater chemistry take place in the downgradient direction in response to several chemical processes. These processes include early concentration by evaporation and congruent dissolution of calcite followed by widespread incongruent dissolution and ion exchange in addition to local oxidation-reduction reactions, gypsum dissolution and saline intrusion. As a result of the above processes, Chalk groundwater follows an evolutionary path from Ca bicarbonate type to Na bicarbonate type. The age of Chalk groundwater was modelled using 14C, δ 13C , 3H, δ 2H and δ 180 . There is a general increase in the groundwater age in a downgradient direction with the oldest water found in N central areas of the basin. Groundwater in the unconfined zones and in areas S of the Greenwich fault is almost entirely of unevolved, modem composition. Carbon-14 modelling suggests that Chalk groundwater in the S basin is generally less than 10000 a old while that in the north is generally between 10000 and 25000 a old. The presence of 3H in concentrations of up to 7 TU in groundwater which yields ages of several 1000 a, however, indicates that mechanisms exist for the rapid introduction of recent groundwater to the confined aquifer. Results of palaeorecharge temperature determinations using δ 2H, δ 180 and noble gas analytical results suggest that significant Devensian recharge did indeed occur in the aquifer. A model of the development of the Chalk recognizes that it is a classic dual porosity aquifer in which groundwater flow occurs predominantly in the fracture system. The upper 50 m of the aquifer was flushed with fresh water during the 2–3 × 10 6 a of the Quaternary and therefore meteoric water largely replaced the Tertiary and Cretaceous marine water that previously saturated the system. Most processes which control the chemistry of the groundwater occur in the matrix where the surface area is exceptionally high. Although fracture flow dominates the flow regime, diffusion from the matrix into the fracture porosity controls the chemistry of Chalk groundwater.

Systematic hydrogeological study of a hypothermal spring (S. Cesarea Terme, Apulia), Italy

A long series of thermo-saline logging has been carried out in wells drilled through the Mesozoic carbonate aquifer from which the sulfur hypothermal springs of S. Cesarea Terme issue. The logging conducted at various timings (i.e. periodically, rapidly sequenced, synchronized with tides and sea conditions), over about 10 years, provides valuable data on the thermal and hydrological regimen of the area. In particular for the inshore zone, both isotherm and thermal gradient trends could be determined, and a close identification of preferential levels through which groundwater discharge takes place was possible. In fact, flow velocity measurements, made by the point diluition method, showed a mostly impervious aquifer except for evident fissured levels through which low-velocity discharge (5–22 cm day −1) takes place. When the sea is low and calm, all levels are influenced by sulfur waters except for the uppermost unconfined zone. When the sea is rough, also owing to the low permeability of the aquifer, a barrier effect against groundwater flow is triggered. Since groundwater is prevented from discharging, it tends to reach deeper permeable levels, thus markedly altering the hydrological and thermal regimen of the deeper sulfur waters. The lithological character of aquifers and their low permeability are confirmed by 222Rn contents (normally 10–15 pCi l −1), groundwater reaching 200 pCi l −1), only at levels where water starts becoming hot. This phenomenon, as supported by all investigations including those on sulfides, occurs only at temperatures exceeding 23°C. Therefore, according to the above investigation, the S. Cesarea springs represent a unique hydraulic model, matching real hydrodynamic situations occurring when surrounding conditions change.

Chlorine-36 in groundwater as a palaeoclimatic indicator: the East Midlands Triassic sandstone aquifer (UK)

The 36Cl contents of groundwaters from the East Midlands Triassic sandstone have been used to deduce rainfall, its chloride content and evapotranspiration during the Late Pleistocene. At this time, the regional precipitation and evapotranspiration would have been reduced relative to the modern and Holocene climate. It is estimated that the infiltration was reduced by about 40 mm · a −1. The method for evaluation of these palaeoclimatic parameters is dependent upon the major Cl − source being a marine aerosol fall-out. This is the general case, but groundwater east of the exploited zone of the aquifer contains chloride which is derived from saline formation water, with a marine Cl −Br/ − ratio, and in equilibrium with the in situ neutron flux. The fall-out of cosmogenic 36Cl at the geomagnetic latitude 56.2°N is evaluated as 30.6 atoms m −2s −1 and is significantly greater than that estimated from the theoretical fall-out curve of Bentley et al. [1]. The ‘bomb peak’ of 36Cl remains entirely within the unconfined zone of the aquifer. The assessment of groundwater residence time from the 36Cl chronometer is shown to be impossible for this aquifer.

AN EXAMINATION OF THE MECHANISMS CONTROLLING GROUNDWATER GRADIENTS IN HYPER‐ARID REGIONAL SEDIMENTARY BASINS1

ABSTRACT: Although evidence of modern recharge in the North African and Arabian sedimentary basin aquifers exists, it is difficult to determine the volume of recharge. Also, from the evidence of regional groundwater gradients, the flow within the aquifers seems to be appreciably greater than one would intuitively expect. A hypotehtical model embodying the characteristics of the aquifers has been used to investigate the likely significance of various possible flow mechanisms. It is shown that while dewatering in the unconfined area can possibly contribute to flows for a considerable period of time, the maintenance of water levels in the unconfined zone must be the result of modern recharge. It is also shown that recharge depths of less than 10 mm per annum are sufficient given suitable aquifer parameters. Results for various combinations of aquifer parameters and configurations are given, including layered aquifers and the effects of restricted oufflows. Comparisons are made using a “bench mark” example.The work indicates that there is little point in carrying out conventional hydrological balance studies in hyper‐arid areas and that, instead, more emphasis should be placed upon good groundwater hydrographic data and modeling.

Trace organic compounds in groundwater: a survey of unpolluted groundwater from three major British aquifer systems

ABSTRACT In view of the increased concern about the occurrence and persistence of organic chemicals in the environment, the Water Research Centre instigated a baseline survey of trace organics in groundwater from representative cross-sections of three major British aquifer systems. A total of 32 public and private supply boreholes in selected areas of the Permo-Triassic Sandstone, Chalk, Lincolnshire Limestone and the Great Oolite aquifers have been sampled. The areas have been chosen with the aim of characterising the organic content of ‘clean’ groundwater from the confined and unconfined zones on each cross-section. The survey was designed to encompass groundwaters of varying ages, from recent recharge in outcrop areas through to ancient waters in deep confined sections. In this way, it was expected that any compounds of anthropogenic origin would be detected only in the younger groundwaters. The total organic carbon content of the groundwaters sampled was generally very low, between 0.2 and 1.4 mg/1; low levels of aromatic halogenated hydrocarbons, which are generally taken to be contaminants, were found in many of the samples at levels above 0.01 µg/1; low levels of aromatic hydrocarbons were often detected. Although the number of compounds detected averaged 20 per sample (range: 7 to 52), few were present at concentrations greater than 0.1 µg/1. Over 100 different compounds were detected altogether but only 75 could be identified positively. Of these, nine occurred in more than half of the samples. There were no obvious differences in either compound types or levels between the confined and unconfined zones of any of the aquifers. Uncontaminated groundwater examined in this survey may be typified as having a TOC level below 0.8 mg/1, and no more than four compounds (none of which are chlorinated hydrocarbons) present at levels above 0.1 µg/1.

A finite-element model for concurrent confined-unconfined zones in an aquifer

The numerical simulation of the situation of concurrent confined and unconfined zones in a groundwater aquifer system is carried out by using the finite-element method with four-sided mixed-curved isoparametric elements. The model is limited to the steady state and is based on Dupuit's assumptions. The results of the model for a test situation indicate that the model can be used for prediction of the occurrence of unconfined regions around over-pumped wells in an originally confined aquifer of which the interfaces between the confined and unconfined regions may be estimated.