Temporal and spatial analysis of nitrate concentrations from the Frome and Piddle catchments in Dorset (UK) for water years 1978 to 2007: Evidence for nitrate breakthrough?

• Published in: The Science of the total environment Vol. 407; no. 1; pp. 507 - 526
• Main Authors: Howden, N.J.K., Burt, T.P.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 15-12-2008; Elsevier
• Subjects: Applied sciences; Aquifers; Calcium carbonate; Catchments; Chalk; Continental surface waters; Correlation; Downstream effects; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; England; Environmental Monitoring; Exact sciences and technology; Freshwater; Groundwater-fed rivers; Groundwaters; Harbours; Hydrology; Kinetics; Laying; Linearity; Long time series; Lowland Chalk catchment; Mathematical models; Natural water pollution; Nitrate trends; Nitrates; Nitrates - analysis; Pollution; Pollution, environment geology; Rivers - chemistry; Samples; Sands; Seasons; Statistical analysis; Statistical methods; Time Factors; Time series; Trends; Water Movements; Water Pollutants, Chemical - analysis; Water treatment and pollution; England; United Kingdom; Great Britain; Europe; British Isles, England, Somerset, Frome; British Isles, England, Dorset; British Isles, England; British Isles, England, Dorset, Poole Harbour; Nitrate trends; Long time series; Lowland Chalk catchment; Groundwater-fed rivers; Nitrogen compounds; Geochemistry; Environmental factor; Nitrates; Spatial variation; Time variation; Hydrochemistry; Lithology; Seasonal variation; Trend analysis; Watershed; Surface water; Stream; Water pollution; Concentration distribution; Ground water
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2008.08.042

This paper analyses time series of nitrate concentration for 35 sites throughout the Frome and Piddle catchments in southern England for the period 1978–2007. The Piddle catchment lies wholly within the Chalk aquifer in its upper reaches, whereas the headwaters of the Frome are a mixture of Chalk and other lithologies; both rivers flow across Palaeogene sands and clays in their lower reaches, before discharging into Poole Harbour. At each site sources of observed variation arise from differences between sample month (seasonality) and year (underlying trend). Considering water year mean concentrations, there is a strong and highly significant positive correlation with year for all but two sites. The annual rate of increase tends to be higher in the Piddle than the Frome, and in both catchments the annual rate of increase tends to decrease downstream; both these effects relate to the degree of groundwater influence. In addition to the strong upward trend, there is also a clear seasonal cycle in nitrate concentration at most sites; amplitude generally increases downstream. Laying aside the assumption of linearity, solute breakthrough curves (S-curves) are fitted to 34 of the 36 nitrate time series. Results show that application of S-curves is an appropriate description of changes in annual mean concentrations over time and provides improvements over linear models in most cases. The interpretation of rising nitrate concentrations in these groundwater-dominated catchments moves from a statistical to a hydrological process base, providing valuable insight into lowland catchment function.