Grab Sampling
Source: Stubbs et al. 1987
Where it is not possible to collect cores, samples will generally be taken by van Veen grab with a sampling area of 0.1 m2. Grab sampling is suboptimal for sediment geochemistry or for meiofauna sampling because the stratified structure of the sediments is usually compromised by the sediment collecting mechanism of the grabs. However, because van Veen grabs are affordable and are easy to use from a small boat, these will most likely be used by farmers and regulators monitoring sediments below fish farms.
Benthic sampling using a Van Veen grab
A 0.1 m2 modified Van Veen grab may be used to collect sediment samples for physical, chemical, and infaunal analysis (Figure 2) (Stubbs et al. 1987). The grab may be galvanized, stainless steel, or Teflon-coated. All surfaces of the grab must be clean and free of rust. Either single or tandem van Veen grabs are acceptable.
Grabs will be used if:
* a large (surface area and volume) sample is required, e.g. for macrofauna
* it is not possible to sample with cores by divers or by using a corer
However - grabs are not suitable for surface layer sampling (e.g. for meiofauna or some other analyses) because they disturb and "blow off" the surface layer
Grab sampling procedures
Prior to deployment, the grab is cocked with the safety key in place. The grab is then hoisted over the side, the safety key is removed, and the grab is lowered at 2 m/sec until it is 5 m above the bottom. From this point, it is lowered at 1 m/sec to minimize the effects of bow wave disturbance. After bottom contact has been made (indicated by slack in the lowering wire), the tension on the wire is slowly increased, causing the lever arms to close the grab. Once the grab is back on board, the top doors are opened for inspection.
Criteria for acceptable grab samples
Upon retrieval of the grab the acceptability of the sample must be determined. Acceptability is based upon two characteristics of the sample: sample condition and depth of penetration. Sample condition is judged using criteria for surface disturbance, leakage, canting, and washing (Figure 3). Acceptable sample condition is characterized by an even surface, with minimal surface disturbance, and little or no leakage of the overlying water. Heavily canted samples are unacceptable. Samples with a large amount of "humping" along the midline of the grab indicating washing of the sample during retrieval are also unacceptable. While some humping will be evident in samples from firm bottoms where penetration has been poor, this is due to the closing action of the grab and is not evidence of unacceptable washing.
Figure 2. Modified Van Veen grab sampler recommended for marine recieiving-water monitoring programs in Southern California: a) cocked position; b) tripped position (modified from Stubbs et al. 1987)
Figure 3. Examples of acceptable and unacceptable grab sample condition
If the sample condition is acceptable, the overlying water is drained off and the depth of penetration determined. The overlying water in grabs intended for infaunal samples may be drained but all drained water must be captured for screening with the sediments (see Sample Processing below). Extra caution should be taken to drain the overlying water from the grabs for chemistry and toxicity samples. It is recommended that siphoning or decanting be employed for these grabs to avoid disturbance and loss of the surface sediments.
It is important to get the best sample possible. For infaunal samples, sediment penetration depth must be at least 5 cm; however, penetration depths of 7-10 cm should be obtainable in silt (fine sand to clay). The depth of penetration is determined by insertion of a plastic ruler vertically along the grab midline and measurement of the depth of sediment to the nearest 0.5 cm.
Sediment Description
The field description of sediments is required following measurement of penetration depth. The sediment description should encompass the following:
- at minimum the sediment should be characterized as being shell hash, gravel, sand, or mud (silt and/or clay), but if possible, the following should be done:
- surface colour and colour change with depth as a possible indicator of redox state;
- smell: sulfide (the odor of H2S or rotten eggs), oily (the odor of petroleum tar), or humic (a musty, organic odor). Typically, sediments will have no particular odor.
- General sediment colors (e.g., black, green, brown, red, yellow)
- description of sediment types, including important notes, e.g., the occurrence of concretions, loose algae, etc.
Precise position fixing during sampling is essential. The position and the depth should be controlled and documented during station work.
References
Stubbs, H.H., D.W. Diehl, and G.P. Hershelman. 1987. A van Veen grab sampling method. So. Calif. Coastal Water Res. Proj., Long Beach, CA., Tech. Rep. No. 204.
