Effects of water sample preservation and storage on nitrogen and phosphorus determinations: implications for the use of automated sampling equipment

Abstract

The effects of freezing, acidification, refrigeration and extended storage without refrigeration on measured concentrations of forms of nitrogen and phosphorus were assessed in water samples taken from polluted and unpolluted streams in dry and wet weather. Emphasis was placed on likely changes associated with the use of automatic samplers in field conditions. The response of measured nutrient concentrations to different preservation methods and to extended storage without preservation varied widely between samples. For polluted streams where nutrient concentrations were high (ammoniacal nitrogen >0.1 mg l⁻¹; oxidised nitrogen, total Kjeldahl nitrogen, total and filterable phosphorus >1 mg l⁻¹), there was generally little proportional change in nutrients other than ammoniacal nitrogen after up to 6 d of storage without preservation. Where nutrient concentrations were low, up to 90% of ammoniacal nitrogen, 50% of oxidised nitrogen, 84% of total Kjeldahl nitrogen and 67% of total phosphorus was lost after 6 d. If samples cannot be retrieved immediately, automatic samplers should be refrigerated where possible. Acidification is a suitable alternative for preservation of nitrogen. Samples for filterable phosphorus should be filtered as soon as possible after collection. In some circumstances, unpreserved samples may provide an acceptable level of accuracy for evaluation of broad temporal trends or estimation of nutrient loadings.

Introduction

Automatic samplers (portable pumping samplers) are increasingly being used for water quality monitoring of rivers and streams (Thomas and Eads, 1983; Ferguson, 1994). These devices allow a number of samples to be obtained at pre-set intervals, for example over 24 h, without the commitment of personnel required for frequent or extended manual sampling. In addition, they can be interfaced with flow-measuring devices to sample in response to discharge changes, such as sudden rises in flow following rainstorms. In these circumstances, prompt manual sampling can be logistically difficult. A potential disadvantage of automatic samplers is that for logistical reasons it may be some time before sites can be visited to retrieve samples and return them to the laboratory for analysis. During this interval, there is the potential for physical, chemical and biological processes to change the water quality characteristics of the sample (cf. Johnson et al., 1975; Klingaman and Nelson, 1976; Lambert et al., 1992).

In this study the effects of various techniques of preservation and storage on measured concentrations of forms of nitrogen and phosphorus in water samples were assessed. These water samples were taken from polluted and unpolluted streams in dry and wet weather. Emphasis was placed on likely changes associated with the use of automatic samplers in field conditions.