WSP05034 - Laserfiche WebLink

Home Browse Search

i'B60 Onsite Measurements of Water-Quality Characteristics Certain water-quality characteristics can change rapidly when a sample is removed from a stream or aquifer. These characteristics must be measured onsite, rather than wait for laboratory analysis. Onsite measurements neces- sary in salinity studies are temperature, specific conduc- tance, pH, and alkalinity. Methods for making these measurements are generally described by Wood (1976): (1) Measure surface water temperature in the stream and in the composite sample and measure ground water tefuperature in the pumped sample. (2) Measure specific conductance and pH in the com- posite or pumped samples. Because specific conductance is temperature dependent, these measurements should be made simUltaneously" and as soon as possible after the sample has been collected. and capped tightly to exclude all air. The bottle should be stored in ice until the titration can be made. An accurate onsite pH value is crucial to water quality and related geochemistry analysis. Probes and meters used for pH measurement are sensitive instruments and must be handled carefully to ensure accuracy. It is very important to follow the manufacturer's instructions for transport and storage, cleaning, calibration, and operation. The pH electrode and meter should be calibrated at least once a day during use with pH buffer solutions over an ap- proximate pH range of 4 to 10. After the probe and meter have been calibrated over this range, check the meter with a single pH buffer solution prior to each sampling. This one-point check is made with apH buffer so.lution relative- ly close to the suspected pH of the water sample. ' Alkalinity is determined by acid titration using a sub- sample of the composite or pumped sample. If the titra- tion cannot be made at the time of sampling, it should be made within 24 hours at the field office or motel. The necessary subsample can be withdrawn from the churn or bucket into a sample bottle that must be rilled completely Sample Filtration and Preservation Accurate measurement of certain dissolved chemical con- stituents in natural water requires fIltration of the sample in the field. This halts further reaction with suspended sediments, The recommended field filtration unit is a backllush filter plate used in conjunction with a peristaltic pump. The pump provides power to force the water to flow through the fUter system. The ftIter system should be fitted with a nitro-cellulose (0.45 micrometer pore size) fIl- ter that removes most of the suspended material from the sample. After onsite measureinents have been completed and unfil- tered subsamples have been removed from the composite or pumped sample, the filtered subsamples can be ' withdrawn by pumping directly out of the churn or bucket. To minimi7" contamination from the ftIter, at least 50 mil- liliters of water should be allowed to pass through the fIl- ter system prior to collection in sample bottles. After each use, the' fUter plate should be thoroughly rinsed with dis- tilled water. Proper sample preservation and storage minimi7"S chemi- cal reactions in a water sample once it has been collected and bottled. This keeps the water as close as possible in the form of the original sample. For dissolved cations (Ca+ +, Mg+ +, K+, Na+) analyses, concentrated nitric acid is added to the sample container to ensure that ions stay in solution and do 'not adsorb onto container walls. Storing samples in ice reduces chemical activity. The laboratory where the samples will be analyzed should be asked to provide recommendations for sample preservation and shipment, as well as necessary materials, such as sample bottles, acid preservatives, and shipping containers. 34 35