Analytical Methods:PH Monitoring

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Degradation processes linked to soil pH
Soil pH is a measure of acidity and as such is a key factor in the preservation and degradation of many material types. There is no one soil pH value that is optimal for the preservation of all material types. Some materials are preserved best in alkaline (pH 7.5-14) conditions such as bone and shell, and others that are preserved best at acid pHs (pH 0-6) such as pollen.

More information on optimal preservation conditions for different material types can be found here.

Changes in soil pH in either direction from current levels are potentially of concern. However, soil pH can change relatively rapidly in response to external factors such as acid rain, land management practices such as liming and changes in vegetation. The response of a soil to external forcing of pH (atmospheric deposition, liming etc.) differs according to the prevailing soil conditions. Recent research has shown the negative effect of acid rain deposition on buried archaeological artefacts (Nord et al. 2005) although a generalised increase in soil pH has been reported for the period between 1978 and 1998 in UK soils following successful measures to tackle acid rain (Haines-Young et al., 2000).

Approaches to monitoring soil pH
Bulk soil pH is likely to be most relevant to monitoring cultural heritage preservation in the oxidised soil zone. Groundwater pH may also be monitored in waterlogged areas through the use of dipwells.

There are two main approaches to monitoring in-situ analysis and point sampling. The first involves installing probes and ideally a datalogger to measure pH at regular intervals. The latter involves making repeat visits to the site and removing soils and / or water samples for analysis in the laboratory. In-situ measurements may be preferred when large numbers of measurements are required on a relatively short repeat time. However, probe arrays can be expensive (this should be compared to labour costs for repeat visits and laboratory analysis), they may be subject to vandalism, and the process of installing the probes may itself be damaging to the site.

Sampling
Soil pH can vary greatly over very short distances depending on deposit composition. The number of sampling points needs to reflect the soil and archaeological variability over the site. On an entirely uniform site Smit et al. (2006) recommend a minimum of three sampling points. Sampling may also need to take account of vertical variation as changes may be initiated from above (atmospheric deposition) or below (groundwater). The frequency of measurements needed will be determined by the process that is being monitored. Monitoring changes in response to a particular activity such as construction for example may require a period of pre-construction monitoring, a period of relatively intense monitoring during and following construction, annd a period in which the frequency of measurements gradually trails off following a period of stabilisation post-construction.

Related techniques

 * Field soil pH analysis
 * Laboratory soil pH analysis

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