Hawkesbury-Nepean River Environmental Monitoring Program

5. Trends in water quality and quantity
Water quality in rivers is largely a function of land use and catchment geology as well
as in-stream processes. A wide range of human-related inputs to river systems can
affect in-stream processes and can be detected by using water quality analyses. For
example, high nutrient levels can stimulate algal blooms and toxic chemicals can
severely affect aquatic plant and animal communities.
One of the limitations of water quality sampling and analysis in the Hawkesbury-
Nepean is that it is generally undertaken monthly, so that short-term changes may
not be easily detected without further additional investigation. It also means that it
may take some time (years) before trends in water quality become evident in the
routine monitoring program. Water quality sampling also focuses on only a subset of
indicators, as it is both impractical and prohibitively expensive to test for all known
chemical compounds that could be detrimental to river health. Appropriate water
quality indicators are extensively discussed in the ANZECC/ARMCANZ (2000)
Guidelines (particularly Volume 2, Section 8.2) and the indicators used in the
Hawkesbury- Nepean Monitoring Program are considered to be an appropriate
subset of water quality indicators to monitor. More specialised programs are required
where specific contaminants are the primary focus of concern (e.g.
pesticides/herbicides or endocrine disruptors).
Longitudinal trends in water quality
Sites that have been extensively sampled in the past for water quality are identified in
Figure 5.1. Figure 5.2 illustrates the changes in various analytes as you move from
the bases of the dams downstream to the mouth of the Hawkesbury River at Broken
Bay. These figures include historic data from a more extensive network of sites than
is being monitored or identified for monitoring in the current Program. They also
contain a number of tributary sites that are likely to be unrepresentative of water
quality in the main channel of the Hawkesbury-Nepean River. Despite this, they are
useful in identifying specific tributaries (e.g. South Creek, Rickaby’s Creek, Grose
River) where water quality is significantly different from that in the main stem of the
river and can contribute (both negatively and positively) to overall water quality and
ecosystem health in the Hawkesbury-Nepean River system. The figures are based
on long-term medians, and the data file from which these graphs have been
produced is included in Table A1.2 (Appendix 1). Recent changes (e.g. nutrient
reductions at STPs) are likely to reduce some of the highest spikes, and this can be
seen in the gradual decreases in the time series of nutrient concentrations at many of
these sites (See Water quality trends using statistical models below).
10 Hawkesbury Nepean River Environmental Monitoring Program: Final Technical Report