River Poulter - Hydrology

Hydrology

The source of the river Poulter is in a region where the underlying geology is an extensive water-bearing porous rock structure called the Magnesian Limestone aquifer. Magnesian limestone is so named because if contains quantities of the mineral Dolomite, which is rich in Magnesium. Where this rock reaches the surface, springs often occur, and the Poulter starts at such a spring. Further to the east, the Magnesian Limestone is covered by another layer of porous rock called the Triassic Sherwood Sandstone aquifer, which is the major geological component of the area. Continuing eastwards, both are then covered by a layer of Mercia mudstone. The groundwater levels in these aquifers affect whether they supply water to the river system, or take water from it, and levels are lower than they have historically been, as a result of the extraction of groundwater, particularly for public water supply, and by fracturing of the aquifers as a result of subsidence caused by deep coal mining.

Above Cuckney, where the Environment Agency have a gauging station which records flows in the river, the catchment for the River Poulter covers some 12 square miles (31 km2), which has an average annual rainfall of 24 inches (610 mm) (based on figures from 1961 to 1990). About a third of this finds its way into the river. Water quality is good. The Environment Agency use a six stage rating scale, from 'A' to 'F', called the General Quality Assessment, to classify rivers. 'A' on the GQA is the best quality of water, while 'F' is the poorest. Factors which affect the quality are levels of ammonia, levels of dissolved oxygen and the Biochemical oxygen demand (BOD), which measures the amount of dissolved oxygen needed by organisms to break down organic matter in the water. These factors are generally worse when the water is discharged from sewage treatment processes, and tend to be worse in summer, when such discharges make up a greater proportion of the total flow in the river. The upper Poulter, above Welbeck Lake, is rated at 'B' on the GQA scale, despite the fact that the flows are augmented by the outflow from Langwith Sewage Treatment Works, which contributes around 1.2 Ml/d (megalitres per day) and by water from Langwith fish farm, which can be up to 0.9 Ml/d, but which is seasonal in nature.

The quality of the water remains good through the length of the river. Figures for the final section before it joins the River Idle show that the levels of ammonia and dissolved oxygen are those which would be expected in a natural ecosystem. Between 1991 and 2006, the BOD has been reduced by more than 50 percent, but there is still room for improvement. Levels of phosphates and nitrates, which are affected by agricultural activity, are quite high, but some improvements have occurred. Water quality is also measured by two biological indexes. These are NTAXA, which relates to the total number of invertebrate species found in samples of the water, and ASPT, (the Average Score Per Taxon), calculated from the numbers of specific invertebrates which are graded according to their sensitivity to organic pollution. The assessment for the years 2004 to 2006 rated the final section as grade A (very good) on the first index, and grade B (good) on the second. This indicates that the water is not far short from unpolluted.