Lakes sampling frequency

Sediment samples should be collected at an appropriate frequency, which will have to be defined on a local basis, taking into account die sedimentation rate in the studied water body and hydrological condidons (e.g. flood events). Typical sampling frequency will vary from once every 1-3 years for large rivers or estuaries that are characterised by high sedimentation rates, to once every 6 years for lakes or coastal areas with very low sedimentation rates. 

The minimum, maximum, mean, and median m-cresol concentrations for 2 unremarked ambient surface water data points are 16.0, 23.0, 19.5, and 16.0 pg/L (STORET 1989). m-Cresol was detected with a frequency of occurrence of 0.9% in surface water (CLPSD 1988). In addition, m-cresol was listed as a contaminant of the St. Joseph River in the Lake Michigan Basin (Great Lakes Water Quality Board 1983). m-Cresol was detected in freshwater samples from Spirit Lake, Washington, on August 7, 1980 at unreported concentrations (McKnight et al. 1982). 

Hexabromobiphenyl was detected (detection limit 6.6 g/kg) at a frequency of 8 57% in human adipose tissue samples from six Canadian Great Lakes municipalities in 1984 (Williams et al. 1988). The concentration of 2,2, 4,4, 5,5 -hexabromobiphenyl in adipose tissue samples pooled from tissues of the general population of the conterminous Unites States ranged from 1 to 2 g/kg (Lewis and Sovocool 1982). PBB levels in the adipose tissues of 15 quarantined dairy farm residents in mid-Michigan (where the mix-up involving FireMaster BP-6 occurred) ranged from 0.104 to 174 mg kg (Humphrey and Hayner 1975). 

In general, samples may be obtained from faucet outlets, at different points in pipe systems, from the surface of rivers and lake waters, and at different depths. The most important consideration is that the frequency and duration of sampling be sufficient to obtain a representative and reproducible sample. In some cases, composite samples may be used, in which individual samples taken at frequent intervals are combined. 

Figure 12. Results of particle size analysis of a sample from West Basin Lake. Australia. A Histogram B Cumulative frequence curve plotted using arithmetic- scales C Cumulative frequency curve plotted using probability-0 scales.

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