Laboratory Dissolution Studies of Rocks from the Borrowdale Volcanic Group (English Lake District)

Laboratory studies were conducted to investigate the rates ofrelease of Na, Mg, Al, Si, K, Ca and Fe from 10 samples of metamorphosed igneous rock of the Borrowdale Volcanic Group (BVG), consisting mainly of plagioclase feldspars, chlorite andquartz, with minor amounts of hornblende, epidote and carbonates.Experiments were performed using freshly-prepared rock pieces (ca. 12 mm cubes), in batch mode, but with frequent changes of the solution phase so that accumulation of dissolved solutes wasminimised. The initial element release rates were relatively high, but declined with time to reach approximate (pseudo-) steady state conditions. Element release rates declined withpH in the pH range 2–7, the dependence upon pH decreasing in the order Al ～ Fe > Si > Mg ～ Ca > K ～ Na. Thepseudo-steady state rates for Si were comparable to steady state values for single minerals. Calcium showed the greatestsample-to-sample variation (more than 100-fold). Rock dissolutionwas far from stoichiometric, with Ca and Mg being present in thesolution phase in proportions greater than in the rock, and Al and Si in lower proportions. In some samples the release rates ofMg and Ca were equal, while in others the Ca rate was up to 100times that of Mg. The rate of release of Ca is correlated with the carbonate content of the rocks. The results indicate that the main source of Ca in the faster-dissolving samples is calcite, while the highest release rates of Mg are consistent with dissolution of dolomite. Aluminium, Si and Fe are probablyreleased predominantly from chlorite. The results are discussed in terms of the supply of base cations to stream waters drainingcatchments underlain by rocks of the BVG.     

Dissolved organic carbon in soil solutions: a comparison of collection methods

A field study was undertaken to compare dissolved organic carbon (DOC) concentrations in soil solutions obtained with three different sampling methods over a range of soil types. The sampling devices used were a tension‐free collector, a tension Prenart collector and a tension Rhizon collector. Samples were collected fortnightly for a year at seven sites in northern England, each collection being replicated three times. The soil solution DOC ranged from 1.3 g m^?3 in an acid ranker to 34.7 g m^?3 in a peat. The DOC concentrations obtained with the three methods correlated reasonably well (r² = 0.6–0.8) but with an indication of bias, as the best fit line differed from the 1:1 line. The tension‐free collector gave generally higher DOC concentrations except at very low concentrations (in the acid ranker soil). The DOC concentrations measured with the tension‐free collectors were significantly (P < 0.05) higher than those obtained with Prenart and Rhizon collectors at four and six sites, out of seven, respectively. Subsequent laboratory tests on tension‐free collected samples showed no DOC loss on filtration through 0.1 and 0.22‐μm membranes, whereas a significant loss of DOC occurred when tension‐free collected samples were subsequently passed through Prenart and Rhizon collectors, indicating a probable sampling artefact with the tension devices. The difficulties of acquiring representative soil solution samples are discussed, together with the advantages and disadvantages of tension and tension‐free methods.