Factors Determining Water Chemistry

The aqueous solubility of a substance is the main chemical property determining the substance’s concentration in water environments. This property is affected by intrinsic factors of the aqueous solution, such as redox, pH, and adsorption for example. Extrinsic factors, such as geology, geomorphology, climate, atmosphere deposition (wet and dry) and gas exchange, and human activities, also affect how much of the substance is effectively dissolved in the aqueous environment. Besides, the extrinsic factors may act differently for surface or ground waters.

Global river chemistry shows an enormous range in concentrations, ionic ratios, and proportion of ions in cation and anion sums spanning several orders of magnitude. It is unrealistic to define any kind of average of global water chemistry composition based on the large river basins. Besides, Meybeck notes that river chemistry is very sensitive to alteration by human activities, and that, in the northern hemisphere, it is now difficult to find a medium-sized basin not significantly impacted by human activities. Different chemical elements and species have different degrees of chemical reactivity, such as precipitation and incorporation into mineral phases, bonding to organic functional groups, and adsorption onto inorganic solid surfaces.

These factors and the amount of colloids and suspended solid material in natural waters determine much of the element dispersion and concentration decay from source. For example, kinetic factors explain why most natural waters are supersaturated relative to amorphous silica in silicate environments. Trace metals are often undersaturated relative to most common metal solid phases due to adsorption.

This has been acknowledged in trace metal concentration differences due to the pore size filters used in water collection for analysis, in which colloidal material plays an important role in the speciation of those metals. Nriagu and Pacyna estimated that human-induced mobilization of several trace metals far exceeds the natural fluxes, and enrichment factors in the range of 3-24 are reported for such elements as As, Cd, Pb, Se, and Hg.

Groundwaters are less perturbed by some of the extrinsic factors previously mentioned. Besides, other factors must be considered to understand and predict the chemistry of groundwaters: aquifer heterogeneity and physical properties, such as porosity, permeability, and transmissivity, as well as the recharge rate. These properties determine the residence time of groundwaters which can typically span hundreds to tens of thousands of years.

The residence time also determines the characteristic response time of the groundwater system to external perturbations, which can be short-term inputs as human activities usually are, or long-term forcings such as climate change, or sea level rise. Residence time is also a factor that influences the groundwater chemistry.

Groundwaters with long residence times in aquifers tend to become closer to chemical equilibrium with the minerals of the host rocks. In turn, waters with short residence times are usually out of equilibrium because the slow kinetics of silicate dissolution. However, situations where rapid ongoing dissolution and precipitation reactions are taking place result in relatively unaffected groundwater chemistry, irrespective of flow rate and direction. As will be presented next, these aspects are fundamental and should be taken into account for the definition of background concentration of elements in diverse water systems.