Continuous Monitoring

Continuous monitoring is another option for a limited number of parameters such as pH, temperature, dissolved oxygen (DO), conductivity, and TOC. Reliability, accuracy, and cost vary with the parameter. Continuous monitoring can be expensive, so continuous monitoring will usually only be an appropriate requirement for the most significant dischargers with variable effluent. The environmental significance of the variation of any of these parameters in the effluent should be compared to the cost of continuous monitoring equipment available. The regulations concerning pH limits allow for a period of excursion when the effluent is being continuously monitored (40 CFR 401.17). Continuous monitoring or labor‐intensive periodic monitoring by grab sampling is necessary where pH excursions are allowed.

Sample Preservation and Holding Times

Preservation techniques ensure that the sample remains representative of the wastestream at the time of collection. Since most pollutants in the samples collected are unstable (at least to some extent), this instability requires that the sample be analyzed immediately or that it be preserved or fixed to minimize changes in the pollutant concentration or characteristics between the time of collection and analysis. Because immediate analysis is not usually possible, most samples are preserved regardless of the time of analysis. This preservation must take place as soon as possible after collecting the sample. This means that preservation must take place in the field (CWA 1972 40 CFR 136.3). The most common procedures used for preserving samples include icing, refrigeration, pH adjustment, and chemical fixation. When chemical fixation is used, the chemical preservative must be added before the samples are transferred to the laboratory. Likewise, refrigeration should be supplied immediately upon taking the sample. For many samples, if preservation is not appropriately used, bacteria can quickly degrade certain pollutant constituents (e.g. phenols and phosphorous). Other constituents such as cyanide and sulfides may volatilize or in the case of hexavalent chromium may react to form different chemical species. Proper preservation and holding time for each parameter is essential for the integrity of the monitoring program. Problems may be encountered, however, when 24‐hour composite samples are collected. Since sample deterioration can take place during the compositing process, it is necessary to preserve or stabilize the samples during the compositing in addition to preserving the aggregate sample before shipment to the laboratory. Preservation techniques vary depending on the pollutant parameter that is to be measured; therefore, the inspector must be familiar with the 40 CFR part 136 preservation techniques to ensure proper sample handling and shipment. It is important to verify that the preservation techniques for one parameter do not affect the analytical results of another parameter in the same sample. If this is a possibility, then two discrete samples should be collected and preserved independently. Sample preservation should be provided during compositing, generally by refrigeration to less than or equal to 6 °C (or icing) as required (CWA 1972 40 CFR 136.3).

Refrigeration is the most widely used preservation technique because it has no detrimental effect on the sample composition and does not interfere with any analytical methods. Refrigeration suppresses biological activity and volatilization of dissolving gases and organic substances. This technique is used at the start of sample collection in the field and during sample shipment, and continued until the sample is analyzed by the laboratory. Sample temperature should be verified and recorded by the inspector. This is particularly important if the analytical results are to be used in an enforcement action.

In addition to preservation techniques, 40 CFR part 136 indicates maximum holding times. Times listed are the maximum times between sample collection and analysis that are allowed for the sample to be considered valid. Some parameters have separate holding times from the time of sample collection to extraction/preparation, and from extraction/preparation to analysis. A composite wastewater sample becomes a sample when the last aliquot is collected. A grab sample holding time begins when the sample is collected. At that point, holding time limitations begin. A list of preservation methods and holding times are provided in 40 CFR 136.3 (CWA 1972). Inspectors should consult 40 CFR part 136 for the most current and detailed specifications. These sample preservation procedures and holding times were selected by the EPA because they retard sample degradation and minimize monitoring costs by extending holding times as long as possible.