Sources and ages of dissolved organic matter in peatland streams: evidence from chemistry mixture modelling and radiocarbon data

Monitoring data over the period 1994-2007 were analysed for three streams (Cottage Hill Sike, CHS; Rough Sike , RS; Trout Beck, TB) draining blanket peat underlain by glacial clay and limestone-rich sub-strata at Moor House (Northern England). Dissolved organic carbon concentration, [DOC], showed complex relationships with both discharge and calcium concentration, [Ca]. A model based on [Ca] was constructed to simulate stream [DOC] by mixing dissolved organic matter (DOM) from shallow peat, quantified by measured [DOC] (15-30 mg l-1) in peat porewater, with DOM assumed to be present at a constant concentration (c. 5 mg l-1) in groundwater. A temperature-based adjustment to the measured porewater [DOC] was required to account for relatively low streamwater [DOC] during winter and spring. The fitted model reproduced short-term variation in streamwater [DOC] satisfactorily, in particular variability in RS and TB due to groundwater contributions. Streamwater DOM is largely derived from surface peat, which accounts for more than 96% of the total DOC flux in both RS and TB, and 100% in CHS. Model outputs were combined with streamwater and porewater DO14C data to estimate the 14C contents, and thereby the ages, of DOM from peat and groundwater. The peat-derived DOM is five years old on average, with most of it very recently formed. The derived age of groundwater DOM (8500 years) is comparable to the 4000-7000 years estimated from the DO14C of water extracts of clay underlying the peat, suggesting that the clay is the source of groundwater DOM.