| This paper examines the potential for calcium
carbonate to reduce phosphate pollution in freshwaters by co-precipitation, a
process known as a "self cleansing mechanism". Calcium carbonate
saturation levels and phosphate concentrations (SRP - soluble reactive
phosphate) across the major eastern UK river basins are examined to test for
solubility controls. The study shows that calcite saturation varies for each
catchment as a function of flow and biological activity rather than by direct
regulation by SRP. Indeed, there is no evidence, for any of the rivers studied,
that calcite solubility controls hold. However, for groundwater and
groundwater-fed springs in the Chalk of the Thames basin, calcite saturation is
observed with associated low SRP levels. A self-cleansing mechanism may well be
operative within the Chalk due to two factors. Firstly, there is a high
potential for nucleation on the calcite micro-crystals in the aquifer. Secondly,
there are within aquifer reactions that remove the calcite nucleating inhibitors
(SRP and dissolved organic carbon, DOC) to levels lower than those occurring
within the rivers do. These inhibitors enter the catchment at very high
concentrations in association with agricultural pollution (fertilizer
application and animal slurry) and household contamination (e.g. sewage sources
from septic tanks). Under low flow conditions, when the saturation index for
calcite is at its highest, so too is the concentration of the nucleation
inhibitor SRP. Companion work shows that calcite precipitation can occur at the
water-sediment interface of the river and this may involve SRP removal. The
data, as a whole, define an apparent bound for calcite solubility control where
in the presence of nucleating centres, SRP must be less than 4 mM-P l-1 and DOC
must be less than 150 mM-C l-1: a condition that does not seem to pertain
within most UK rivers.
Keywords: calcite, calcium carbonate, phosphate,
soluble reactive phosphate, dissolved organic carbon, LOIS, UK, rivers,
self-cleansing mechanisms. |