- How do you select the proper isotopic system for dating groundwater?
see “Dating Groundwater with Isotopes,” by Brenda Ekwurzel
- How can I date recent groundwaters (<100 years)?
see text on Tritium (3H) under Hydrogen , Helium-3 and Lead.
- How can I date old groundwater (100 to 1000 years old)?
see text on Carbon-14 and “Locating Recharge Zones with Isotopes,” by Christopher Eastoe
- How can I date very old groundwater (>10,000 years)? (36Cl) (I129)
see text on Helium-4 , Chlorine-36 and Iodine-129
- What type of rocks or sediments has a water interacted with? (Sr, S, Pb)
see Strontium , Sulfur and Lead
- What is the flowpath of a water? (Sr, S, Tucson basin study article)
see Strontium , Sulfur and “Locating Recharge Zones with Isotopes,” by Christopher Eastoe
- What is the source of organic contamination in groundwater?
see text under Carbon
- What is the source of lead contamination?
- What is the source of nitrate in groundwater? (N isotopes)
see text under Nitrogen
- What is the source of salinity in water?
see Strontium , Chlorine and SAHRA work on the Solute Balance of the Rio Grande
Isotope or Chemical Tracer
Geochemical and isotopic tracers and their application in water resources problems.
|GEOCHEMICAL ISOTOPIC TOOL (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 each other 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|