The chemical composition of natural water is derived from many different
sources of solutes, including gases and aerosols from the atmosphere, weathering
and erosion of rocks and soil, solution or precipitation reactions occurring below
the land surface, and cultural effects resulting from activities of man. Some of
the processes of solution or precipitation of minerals can be closely evaluated by
means of principles of chemical equilibrium including the law of mass action and
the Nernst equation. Other processes are irreversible and require consideration
of reaction mechanisms and rates. The chemical composition of the crustal rocks
of the earth and the composition of the ocean and the atmosphere are significant
in evaluating sources of solutes in natural fresh water.
The ways in which solutes are taken up or precipitated and the amounts
present in solution are influenced by many environmental factors, especially
climate, structure and position of rock strata, and biochemical effects associated
with life cycles of plants and animals, both microscopic and macroscopic. Taken
all together and in application with the further influence of the general circulation
of all water in the hydrologic cycle, the chemical principles and environmental
factors form a basis for the developing science of natural-water chemistry.
Fundamental data used in the determination of water quality are obtained by
the chemical analysis of water samples in the laboratory or onsite sensing of
chemical properties in the field. Sampling is complicated by changes in composition
of moving water and the effects of particulate suspended material. Most of the
constituents determined are reported in gravimetric units, usually milligrams per
liter or milliequivalents per liter.
More than 60 constituents and properties are included in water analyses
frequently enough to provide a basis for consideration of the sources from which
each is generally derived, most probable forms of elements and ions in solution,
solubility controls, expected concentration ranges and other chemical factors.
Concentrations of elements that are commonly present in amounts less than a
few tens of micrograms per liter cannot always be easily explained, but present
information suggests many are controlled by solubility of hydroxide or carbonate
or by sorption on solid particles.
Chemical analyses may be grouped and statistically evaluated by averages,
frequency distributions, or ion correlations to summarize large volumes of data.
Graphing of analyses or of groups of analyses aids in showing chemical relation-
ships among waters, probable sources of solutes, areal water-quality regimen, and
water-resources evaluation. Graphs may show water type based on chemical
composition, relationships among ions, or groups of ions in individual waters or many waters considered simultaneously. The relationships of water quality to
hydrologic parameters, such as stream discharge rate or ground-water flow
patterns, can be shown by mathematical equations, graphs, and maps.
About 75 water analyses selected from the literature are tabulated to illustrate
the relationships described, and some of these, along with many others that are
not tabulated, are also utilized in demonstrating graphing and mapping techniques.
Relationships of water composition to source rock type are illustrated by graphs
of some of the tabulated analyses. Activities of man may modify water composition
extensively through direct effects of pollution and indirect results of water
development, such as intrusion of sea water in ground-water aquifiers.
Water-quality standards for domestic, agricultural, and industrial use have
been published by various agencies. Irrigation project requirements for water
quality are particularly intricate.
Fundamental knowledge of. processes that control natural water composition
is required for rational m
Additional publication details
USGS Numbered Series
Study and interpretation of the chemical characteristics of natural water