Stablity Study on COD_Mn, NO₃^-, NH₄⁺ in Groundwater

COD_Mn, NO₃^- and NH₄⁺ are important indicators to nitrite, nitrate and ammonia pollution in groundwater. There are significant differences between geological industry standards and national-international standards about the storage ways and time for these three indexes. Geological industry standards prescribed that when samples are stored at room temperature without adding any reagent, COD_Mn and NH₄⁺ should be tested in 3 days while NO₃^- in 20 days. National and international standards proposed that when samples are kept in dark places or in the refrigerator with addition of sulfuric acid, it is better to detect COD_Mn and NO₃^- within 2-7 days while NH₄⁺ within 1-7 days. In order to analyze the differences of the storage condition of the various types of standards about groundwater, and to make sure getting accurate testing results, stability research of COD_Mn, NO₃^- and NH₄⁺ in groundwater of Guangzhou Region were studied. Under the two kinds of preservation conditions with adding sulfuric acid and adding nothing in groundwater, testing of the three indexes by using new sealed samples at different time. The results showed that all standard methods about water preservation are reliable. Preservation conditions in geological industry standards that are applied to static groundwater are relatively less critical, while preservation conditions in national and international standards that are available to surface water and waste water, which are more complicated, unstable and changeable, are relative more stringency. In addition, in Guangzhou region, under the condition of keeping samples in dark places, COD_Mn and NH₄⁺ could be obtained accurate data both for testing samples with addition of acid for 30 days and without any addition for 5 days. The content of NO₃^- was nearly the same under the two kinds of storage conditions in 30 days. Both two methods are more effective than geological industry and national standards, and it is easier to keep samples in dark places than keep in refrigeration. At last, we proposed that groundwater can be stored without any addition when it is easy and quick to take samples back to the laboratory, alternatively, groundwater can be stored with addition of sulfuric acid.