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Groundwater chemistry in the unsaturated zone
Geoindicator


NAME: Groundwater in the unsaturated zone
(revised by W.M. Edmunds August 2004)

BRIEF DESCRIPTION: Water moves downwards to the water table through porous soils and sediments and, under favourable conditions, may preserve a record of weathering processes, climatic variations (in the Cl or isotopic signature), or human activities such as agriculture(NO3) and acidification (H+). This indicator may be considered as the output from the soil zone and may reflect the properties or change in properties of soils. Rates of downward movement are typically 0.1 to 1.0 m/yr, and a record of individual events (resolution 1-20+ years) may be preserved over a scale of decades or centuries [see groundwater quality; soil quality]: much longer records, up to 105 Tyr, may be available in hyperarid regions. In contrast, records collected over periods of years are needed to establish trends from the monitoring of rivers and streams or groundwater discharge [see groundwater quality; surface water quality]. The unsaturated (vadose) zone is also important as a buffer for the attenuation of acidity, metal content, and some other harmful substances.

SIGNIFICANCE: Changes in recharge rates have a direct relationship to water resource availability. The unsaturated zone may store and transmit pollutants, the release of which may have a sudden adverse impact on groundwater quality.

HUMAN OR NATURAL CAUSE: Both. Depending on land use, the unsaturated zone beneath a site may record either natural inputs from the atmosphere, vegetation, soil or mineral weathering, or the effects of human activities such as agriculture and industrial activity, or regional problems such as acidic deposition.

ENVIRONMENT WHERE APPLICABLE: Temperate and semi-arid regions in particular. In temperate zones, the typical record may extend for 5-50 years, and in semi-arid regions 10-200 years.

TYPES OF MONITORING SITES: Unconsolidated sediments or consolidated porous media (sand, till, sandstone, chalk, calcarenite, volcanic ash) on relatively level terrain (negligible surface runoff). The best records are obtained where the unsaturated zone is some10-30m thick, and where sediments and unsaturated flow are relatively homogeneous.

SPATIAL SCALE: patch / mesoscale

METHOD OF MEASUREMENT: Dry sampling of unsaturated zone sediments is carried out by hollow stem auger (hand or motor-operated), by sampling from dug wells, or by percussion or air-flush rotary drilling. Pore water is extracted from sediments by high-speed centrifuge (drainage or immiscible liquid displacement) or, for non-reactive components such as Cl and NO3, by elution with de-ionized water. For isotopic samples (3H, delta18O, delta2H), vacuum distillation may be used. Measurement of Cl, NO3, SO4 and other representative ions is made by standard methods suitably miniaturized to handle small quantities of water (eg. 5-10 ml).

FREQUENCY OF MEASUREMENT: 5 - 10 year intervals to assess movement of solutes towards the water table.

LIMITATIONS OF DATA AND MONITORING: Relatively homogeneous sediments are required where flow takes place uniformly (“piston displacement”). In dual porosity media, some by-pass flow may occur that needs to be taken into account, as when some contaminant travels relatively rapidly to the water table along fissures. In very dry sediments (<4% volumetric moisture content) it may be difficult to release pore waters, and elution must then be used.

APPLICATIONS TO PAST AND FUTURE: The record in the unsaturated zone may indicate inputs (amounts of recharge, history of recharge or contamination) over periods of 10-100 years, and possibly up to 500 years or more. The resolution of the signal will be related to dispersion. Under favourable conditions (e.g. in semi-arid environments), the unsaturated zone may provide an important terrestrial record of climate/environmental change.

POSSIBLE THRESHOLDS: NA

KEY REFERENCES:

Appelo, C.A.J. & D.Postma 1993. Geochemistry, groundwater and pollution. Rotterdam: Balkema

Cook, P.G., W.M.Edmunds & C.B.Gaye 1992. Estimating palaeorecharge and palaeoclimate from unsaturated zone profiles. Water Resources Research 28: 2721-31.

Geake, A.K. & S.S.D.Foster 1989. Sequential isotope and solute profiling in the unsaturated zone of British Chalk. Hydrological Sciences Journal, 34:79-95.

Edmunds, W.M., W.G.Darling & D.G.Kinniburgh 1988. Solute profile techniques for recharge estimation in semi-arid and arid terrain. In I. Simmers (Ed), Estimation of Natural Groundwater Recharge: 139-157. Higham, MA: Riedel.

Edmunds, W.M. and Tyler, S.W. 2002. Unsaturated zones as archives of past climates: towards a new proxy for continental regions. Hydrogeology Journal, 10: 216-228

OTHER SOURCES OF INFORMATION: Environment and water/hydrological agencies, geological surveys, IAH, IAGC, IAEA.

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Although inputs of pollutants can be monitored in the saturated aquifer, the resolution of data in the unsaturated zone is of high quality and of unique value in providing an archive at annual or decadal scales.

OVERALL ASSESSMENT: Analysis of the chemistry of groundwater in the unsaturated zone is a technique of growing importance in groundwater quality assessment, and the only available fine-resolution means of gaining an instant record of long-term inputs to the hydrological cycle.

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