Controls on the water chemistry of headwater streams : synthesis and regional modelling.
An understanding of the dominant processes controlling streamwater chemistry is critical
for the development of a regional water quality model. The relative importance of a variety
of catchment characteristics in determining streamwater chemistry in the English Lake
District is investigated in this thesis. Fifty-five streams were sampled on a bimonthly basis
over the period May 1996 through March 1997.
Rock samples were analysed using X-ray fluorescence analysis. The major- and traceelement
geochemical data was used to classify the suite into six rock types. The
geochemical data was also used to establish weathering patterns, which suggested that
greywackes and lithic arenites weather the easiest, andesites and dacites exhibit
heterogeneous weathering and the metamorphosed slate and granite are the most resistant to
Forty-one (75%) of the streams can be considered sensitive to acidification on the basis of
their alkalinity and thirty-eight (69%) can be considered sensitive to acidification on the
basis of their calcium concentrations. Statistical analyses showed that flow-weighted
concentrations of alkalinity and base cations were lowest in the catchments with resistant
bedrock, thin or peaty soils, at high altitudes or receiving relatively high loads of sulphur
and nitrogen deposition.
A multiple regression model incorporating some of these factors provided a fairly good
approximation of alkalinity concentrations on a spatial (R2 value of 56%) and temporal
scale (R2 value of 49%). The predicted alkalinity was within 50 Ileq rl of the observed
(simulated) values for 77% of the sites.
Although the model has a fairly good predictive capability, its spatial and temporal
applicability outside the study area is an unknown quantity. To satisfy the needs of policy
makers and the hydrological community, the model needs to predict alkalinity with a high
degree of accuracy in a variety of study areas. This has not been tested as yet, however, the
variables used in the final model are not unique to the Lake District or the 1990's and
therefore the model may prove to be an extremely useful tool indeed.