Detailed water analyses
NOTE: Article from the Geogenic Contamination Handbook
The planning of a water survey can be restricted to the measurement of arsenic and fluoride concentrations, but measuring further parameters may be helpful:
- To identify other contaminants that may be present
- To interpret the causes of geogenic contamination
- To physically and chemically characterise the groundwater
A comprehensive analysis of water composition can be costly, as extensive laboratory analysis is involved. The usefulness of the information must therefore be weighed against the cost. Sometimes a parameter that was not initially considered may later become important. The most important parameters are presented here to aid the reader to decide which water-quality parameters to include in a survey. The composition of groundwater is affected by a combination of processes (Appelo and Postma, 1993) including:
- Weathering, dissolution and precipitation of minerals
- Evaporation and evapotranspiration
- Decay of organic matter
- Selective uptake of ions by vegetation, e.g. potassium and phosphate
- Mixing and dilution
- Ion exchange
All these processes in combination affect ion concentrations in solution, i.e. water composition. In-depth hydrogeological studies would be necessary to fully understand water composition, but in the context of this handbook, it is sufficient to capture the waters that are characteristic for geogenic contamination. Here we focus on sum parameters (Table 4.5), major components (Table 4.6), redox parameters (Table 4.7) and minor components and contaminants (Table 4.8).
The sum parameters pH, Eh (redox potential) and EC (electrical conductivity) can be measured with portable instruments in the field and give a general indication of water quality (Table 4.5). Redox potential, and the related parameter, dissolved oxygen, are liable to atmospheric contamination, making it very important to avoid contact between samples and air.
Major ion chemistry, together with a knowledge of sum parameters, provides an understanding of the type of water that the measurement of the sum parameters alone does not provide. Some examples are:
- The chemical composition of a groundwater with a pH value of 7 to 8 and with calcium as the predominant cation may be controlled by the mineral calcite (CaCO3). In the presence of calcium, elevated concentrations of fluoride would not be expected.
- The chemical composition of a groundwater with a pH value of around 5 to 6 and a low ion content may indicate a crystalline rock environment, for example, granite. Fluoride content could be elevated.
- A neutral to alkaline groundwater (pH range 7 to 8 or above) with high ion content is indicative of arid conditions. High evaporation rates can lead to an increase in salinity (particularly NaHCO3). Under these conditions, calcite (CaCO3) tends to be insoluble, resulting in a low calcium content. Depending on the composition of the source rock, geogenic contaminants might be present.
A groundwater with negative redox potential or no measureable oxygen may contain reduced species (Table 4.7) irrespective of the major ions present. An elevated dissolved organic carbon content might be expected. Under these conditions, soluble reduced arsenic might be present.
The quality of the measurements can be verified by comparing the sum of the cations with the sum of the anions. As aqueous solutions cannot be charged, the two should be equal (“charge balance”). If the calculation shows an excess positive or negative charge (i.e. too few cations or anions), this indicates that the analysis is incorrect or that a parameter is missing. Care has to be taken during the calculation that the units are the same. For example, all units should be measured in milligrams per litre (mg/L). For the charge balance, values then need to be converted to mmol/L by dividing the values in milligrams per litre (mg/L) by the molecular weight of the respective ions. The final step is to multiply the millimolar concentration by the respective charge (z) of the ion, which gives the milli-equivalents of charge per litre (meq/L) for a particular ion. The total charge of the cations and anions is obtained by summing the meq/L as shown below:
∑ cations (meq/L) = ∑ cation concentration (mmol/L) x z (charge)
∑ anions (meq/L) = ∑ anion concentration (mmol/L) x z (charge)
Mole units: One mole is equal to 6.02 × 1023 atoms or molecules of a chemical substance. This number is derived from the number of atoms in 12 grams of carbon (12C). The mole is widely used in chemistry instead of units of mass or volume, because it is a convenient way to express the number of atoms, molecules or other units of reactants or products in chemical reactions. For example, one mole of calcium (Ca2+) will react with 2 moles of fluoride (F-) to form one mole of fluorite (CaF2).
The values represent the equivalents of charge of cations and anions, which – as stated before – should be equal. Agreement to within 10% is excellent. If values differ by more than 20%, the samples must be re-analysed or a missing factor sought. Organic acids can make a significant contribution to the anionic charge in surface and contaminated waters. NOTE: The ion balance is usually limited to the major ions (Na+, K+, Mg2+, Ca2+, Cl-, SO42-, HCO3-, Table 4.6). For most groundwater samples, this is sufficient.
The redox potential is of particular significance in arsenic-contaminated waters, as arsenic is a redox-sensitive element. The measurements of Eh can be substantiated by measuring the concentrations of redox-sensitive species (Table 4.7). The cause of reducing conditions is generally the oxidation of organic carbon (as may be found in young organic-rich sediments) by microbes. The microbes use different oxidising agents in a specific order: oxygen, nitrate, manganese oxides, then iron oxides and sulphate. These are themselves reduced.
Total organic carbon (TOC) or dissolved organic carbon (DOC) content can be an indicator of these processes. Oxic groundwater generally contains ≤ 2 mg DOC/L. It should also be noted that dissolved ammonium (NH4+) is often associated with biodegradation processes and may result from the microbial reduction of nitrate.
The choice of trace metals and metalloids to analyse is dependent on the type of geogenic contamination (Table 4.8).
- For reducing conditions where groundwater may be contaminated with arsenic, it is sufficient to quantify arsenic and possibly iron, manganese and sulphide (noting that sulphide would indicate the absence of arsenic). Arsenic is one of the very few elements (including manganese and iron) that is more soluble in reduced form than in its oxidised state.
- Where fluoride might be expected, usually under oxidising conditions, the analysis of further potential contaminants, including arsenic and uranium, would be beneficial.
Sampling and the preservation of the water samples for the determination of minor and contaminant species should follow guidelines provided by the laboratory (APHA, 2012). The measured values are assessed by comparing them with the WHO Drinking-Water Guideline values or with national standards where applicable.
For references, please visit the page References - Geogenic Contamination Handbook.