Abstract
The South African national water quality database (Water Management System) houses data records from several environmental monitoring programmes, including the National Chemical Monitoring Programme (NCMP). The NCMP comprises an extensive surface water quality monitoring programme, managed by the Department of Water and Sanitation (DWS). The purpose of this technical note is to alert users to a systematic anomaly recently observed in the pH dataset of the NCMP, reflected in an abrupt increase between pre- and post-1990 data records. Although the cause of the anomaly in pH could not be confirmed with high confidence, an inappropriate acid rinse procedure in pre-1990 analytical methods was identified as the most likely cause, based on available evidence. This was supported by the variation in relative sensitivity when comparing the effect on waters with different buffering capacities, i.e., water with low buffering capacity (represented by total alkalinity < 10 mg/L, as CaCO3) showing the largest anomaly, compared with waters of higher buffering capacity (represented by total alkalinity > 30 mg/L, as CaCO3) showing the smallest anomaly. Historical pH data records in the NCMP (i.e. pre-1990), therefore should be used with caution, especially in more weakly buffered systems. The possibility of reconstructing data using a correction factor derived from detailed statistical analyses of the post-1990 pH characteristics at selected sites is a possible solution that could be investigated in future. A key lesson learnt is the need to be diligent in capturing detailed meta-data on sampling procedures and analytical methods in datasets spanning several generations. Availability of such information is critical in order to provide users with a means of evaluating the suitability and comparability of data records in long-term datasets. The DWS includes such meta-data in the current version of the database, dating from about 1995 onwards.
Highlights
The chemistry of freshwater ecosystems is influenced by various factors, including geology, climatic conditions, soils, geomorphology and biological activity (Day et al, 1998; Bluth and Kump, 1994; Huizenga, 2011). pH is used to describe the hydrogen activity in a solution, by defining the acidity or basicity of water, largely determined by factors such as carbonate (CO32-) and bicarbonate (HCO3-) ions released during chemical weathering (Kumbar, 2003; Huizenga, 2011)
The abrupt anomaly between pre-1990 and post-1990 data records across the entire pH dataset in the National Chemical Monitoring Programme (NCMP) pointed to a systematic analytical error
Residual acid from the rinse phase might have caused the error in the pH readings, giving a lower sample pH reading than what it would have been under ambient conditions
Summary
The chemistry of freshwater ecosystems is influenced by various factors, including geology, climatic conditions, soils, geomorphology and biological activity (Day et al, 1998; Bluth and Kump, 1994; Huizenga, 2011). pH is used to describe the hydrogen activity in a solution, by defining the acidity or basicity of water, largely determined by factors such as carbonate (CO32-) and bicarbonate (HCO3-) ions released during chemical weathering (Kumbar, 2003; Huizenga, 2011). Total alkalinity defines the ability of natural waters to neutralise acid within a system when assaulted by the same amount of acid, referred to as its buffering capacity (Weber and Strumm, 1963; Davies and Day, 1998). A screening of the complete NCMP pH database indicated a systematic anomaly in all pH records reflected as an abrupt increase from pre- to post-1990. The purpose of this technical note, is to alert users of the NCMP database of the anomaly in pH data, to pose a potential cause, to test the sensitivity of the anomaly among waters with varying buffering capacity (i.e. using total alkalinity as indicator) and, to highlight implications for future use
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