Determination of characteristic values for physical and chemical lake sediment parameters

The aim of this work is to discuss how characteristic values can be determined for physical and chemical sediment parameters in lakes. It is shown that various physical sediment parameters, e.g., water content, bulk density, organic content, grain size, degree of compaction, and sediment depth, may be related to each other and especially to the water content of the sediments, which may be considered as a key parameter in this context. It is demonstrated that the areal distribution of these physical parameters may be described by the following general formula: Ph 0–1 = Ph K − K L , [(100 − D p )/( D p + K L K ph )] where Ph 0–1 is the given physical sediment parameter from the 0 –1 cm, Ph K is the characteristic content of the parameter in the given lake/basin, K L is an empirical lake constant, K ph is a parameter constant, and D p is the relative water depth ( D p = 100% for the maximum depth). The validity of this relationship is illustrated for five Swedish lakes (Hjälmaren, Vänern, northern Vättern, western Mälaren and Ekoln). The relationship is important since it provides a means for prognosis and determination of physically relevant, characteristic values and not just traditional statistical values, such as various types of means. It is demonstrated: (1) that the distribution of physical sediment parameters depends on the water depth (the relative water depth), (2) that the spread (standard deviation) around the mean is large in shallow waters, where erosion, transportation as well as accumulation of fine sediments may appear, (3) that the spread decreases with increasing water depth, and (4) that the spread is relatively small at all water depths within the zone of accumulation. This allows the determination of a characteristic value at D p = 100% (= D max ). This argument will only be valid in lakes with areas of accumulation. The ‘critical’ water content ( W T−A ) separating areas of accumulation from zones of transportation may be estimated from the relationship W T−A = W K – 10, where W K is the characteristic water content. Characteristic values may be determined in a similar manner for chemical parameters showing a high correlation with the water content, i.e., essentially noncontaminating parameters. For contaminating substances, which are distributed in a typical way in lake sediments in the form of distinct tongues with decreasing concentrations with distance from the polluting source, characteristic values cannot be determined with the mentioned method. However, for contaminating substances, like mercury in the investigated lakes, characteristic values may be determined from the map illustrating the areal distribution pattern as the areal median value.