Sampling in water
Also for water the right technology and sampling strategy depends on the intended aim of the investigation and the local conditions.
- Mixing / sampling points: In contrast to soil, liquid water mixes very easy, which leads to a good homogeneity. Nevertheless also in water there are considerably spatial differences for specific parameters, for example by layering, currents or discharges. To obtain significant samples, these structures must be considered. That means on the one hand, that they must be considered when selecting the sampling points. Under certain circumstances, this may be associated with increased effort, if the sample has to be obtained, for example, from a great depth, or in the middle of a large body of water. The temporal component must be considered as well, as these spatial distributions may be subject to temporal changes. For example by seasonal changes, tidal influences or anthropogenic influences such as opening of weirs or temporary discharges. On the other hand, this means that these spatial distributions shall not be affected during sampling as far as this is possible. Particularly the insertion of measuring and sampling equipment, but also just accessing the sampling point may lead to intermixing. If the same material is used in various places, there may be carry-over when the material is not thoroughly cleaned in between.
- The right material: In order to obtain meaningful samples, it is important that the parameter under consideration is not affected by the sampling equipment. For chemical analysis, this means that every part of the sampling equipment that comes in contact with water must be inert to the substance to be examined. This also needs to be considered for the material of internal components such as hoses or pump diaphragms. Furthermore for some parameters it needs to be ensured that the water is not subject to any temperature fluctuations. Pumps or valves that might get very hot must be avoided in this case. In addition temperature change during transport should be prevented. For example by insulated transport containers. For studies of the sediment load or the microbiology it is essential to have no constrictions and turbulences in the sampling line where material could be retained. For example valve passages or bends often generate limitations in material transport.
- Sediment or open water: Sampling in water doesn`t always mean to extract the sample from the open water body. Sometimes also water samples from the sediment or samples including the deposed sediment for further examination are necessary. In this case adapted sampling technologies allow for a disturbed or mainly undisturbed sampling. Thus aso layers in the sediment can be considered.
- Groundwater: For sampling groundwater bodies there is the particular specificity that these are usually closed towards the atmosphere and not accessible from the outside. Through wells this access is possible. However, thereby also a contact to the atmosphere is established, which can affect the actual parameters. Flushing the monitoring well with fresh inflowing groundwater allows for realistic samples, but may also lead to significant influences on the hydrological regime. Therefore sampling plans for groundwater monitoring wells have to be adjusted to the aquifer. Groundwater wells are particularly prone to intermixing, since the water is easily mixed within the well by inserted technology. It is also important to note that the removal of water from a well can cause influx from other depths of the aquifer as the backfilled space around the well casing may allow for a preferential flow path. When working with groundwater monitoring wells, it requires special carefulness since the entry of foreign substances may affect an aquifer decisively.