Calibration of an early diagenesis model for high nitrate, low reactive sediments in a temperate latitude estuary (Great Ouse, UK)

The description and calibration of a reaction-diffusion model of early diag enesis are presented. Unlike previous models it has been developed for a te mperate latitude estuary (Gt Ouse, UK) impacted by high nitrate concentrati ons (annual mean 700 mu M). Five variables, O-2, NO3-, NH4+, SO42- and S2- are modelled from the steady state distributions of bulk total organic carb on (TOC) (i.e, a l-G model). Three methods for deriving the first order rat e constant, k, for TOC mineralisation are tested: (1) data calculated k val ues [i.e. (depth integrated total mineralisation rate) divided by (depth in tegrated TOC inventory)]; (2) an exponential formulation. k(z) = k(0) e(-al pha z) (k(0) = k at sediment surface, alpha = reactivity coefficient of dec rease, z = depth); and (3) use of separate k values for individual minerali sation pathways. Method a underestimates observed fluxes of solutes across the sediment-water interface (SWI) by up to an order of magnitude. This is due to an inappropriate use of the calculated kin the model. The calculatio n of k yields an overall net Value which implicitly accounts for all factor s acting on mineralisation. Such factors (e.g. oxidant Limitation of organi c decay) are explicitly modelled. Consequently, k is significantly reduced by factors applied to it in the model which have previously been accounted for in the calculation. In Method 2, measured NO3- fluxes are overestimated by up to a factor of 7. To reproduce measured benthic oxygen demands and s ulphate reduction rates, a cannot be simultaneously fitted to the NO3- flux es. The high overlying NO3- concentrations result in model denitrification that cannot reproduce the degree of Limitation that actually occurs. Method 3 reproduces the data (i.e. both stoichiometrically derived mineralisation rates and measured solute fluxes at the SWI) to a high degree (r > 0.99, p < 0.001), but at the expense of increasing the degrees of freedom in the m odel and conceptual simplicity. These results cast doubt over the universal applicability of diagenetic models for estuarine systems exposed to high N O3- concentrations. It is concluded that the use of commonly calculated fir st order rate constants (Method 1) and the frequently used exponential func tion (Method 2) in diagenetic models cannot be relied upon to reproduce obs ervations in high NO3- estuaries. Previous stoichiometric calculations sugg ested that all of the measured ammonium fluxes across the SWI in the Ct Ous e could be accounted for with oxygen, nitrate and sulphate reduction alone. With these latter processes the model (Method 3) underestimates the observ ed ammonium fluxes by up to 44 % at 3 out of 4 sites. This suggests that ot her mineralisation pathways (e.g, nitrate ammonification) are active in the Great Ouse sediments.