1) INERT TRACERS

ROLE AS TRACER

Cl

Master variable: inert tracer in nearly all geochemical processes, use in recharge estimation and to provide record of recharge history

Br/Cl

Use to determine geochemical source of Cl

³⁶Cl

Half life 3.01x10⁵ yr.Thermonuclear production - use as tracer of Cl cycling in shallow groundwater and recharge estimation. Potential value for dating over long timespans and also for study of long term recharge processes. However, in situ production must be known

³⁷Cl/³⁵Cl

Fractionation in some parts of the hydrological cycle, mainly in saline/hypersaline environments may allow fingerprinting

³H

Recognition of modern sources of salinity (half life 12.3)

δ¹⁸O,δ²H

Essential indicators with Cl of evaporative enrichment and to quantify evaporation rates, in shallow groundwater environments. Diagnostic indicators of marine and palaeomarine waters

Noble gases and noble gas isotope ratios

δ³⁴S

Indicator together with δ¹⁸O in the dissolved sulphate to obtain information on salinisation causes, eg dissolution of evaporates or salt water intrusions

CFCs, SF₃₆

Essential indicators with Cl of evaporative enrichment and to quantify evaporation rates, in shallow groundwater environments. Diagnostic indicators of marine and palaeomarine waters

2) REACTIVE TRACERS

ROLE AS TRACER

Major ions ,

Major ion ratios

Ca, Mg, Na,K; HCO₃, SO₄, (Cl) the major ions in groundwater are derived from water rock interaction and their concentrations and ratios (to eachother and to Cl) may be diagnostic of rock type and residence time

Mg/Ca Diagnostic ratio for (modern)sea water

Na/Cl Ratio increase diagnostic of cation exchange reactions

Trace elements and trace/major element ratios

Sr, F, Li and others diagnostic of lithology, residence times etc

Nutrient elements (NO 3 ,K., PO 4 )

Nitrate accumulations may accompany Cl in aerobic arid environments. Nutrient elements characteristic of irrigation returns

⁸⁷Sr/⁸⁶Sr

Additional indicator of source of groundwater source rock with more radiogenic ratios (higher 87Sr derived mainly from igneous rocks

δ¹¹B

Additional indicator of salinity source

¹⁴C, δ¹³C

Radiocarbon activity expressed as percent modern carbon is proportional to groundwater age. Half life 5730. Understanding of carbon geochemistry is essential to interpretation

TOC

Indicator of organic contamination. Natural baseline levels rarely exceed 2 mg/l

δ¹³C

Indicator of organic degradation and calcite dissolution. Also biogenic pollution

δ¹⁵N

Indicator of urban pollution and source of nitrification in urban areas

Organics

Indicator species (eg fatty acids) to characterise marine waters of different age. Pesticides etc diagnostic of irrigation sources of salinity

Dissolved gases

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