Groundwater humic substance concentration

Hydride generation AAS is a well-established technique for the determination of selenium because of its selectivity and sensitivity. The detection limits are improved by concentrating the hydride prior to the transfer to the spectrometer. Cryogenic condensation in a U-tube at liquid nitrogen temperature is a preferred method of pre-concentration. However, selenium hydride is formed essentially only from Se. The selectivity for Se has been combined with various chemical preparation steps to determine the sum of SeIV and Se and the total selenium content of samples. Se concentrations are measured by the difference between the (Se + Se ) and Se contents. The difference between total selenium and (Se + Se ) contents represents the concentrations of Se° and Se2- species. These methods have been used for determinations of selenium species in natural waters. Soluble organic matter in some groundwater samples interferes with the hydride generation determination of selenium. Isolation of humic substances by their adsorption on resins has overcome this problem. 

This chapter discusses and reviews the amount of organic carbon and humic substances in groundwater and the characterization and origin of humic substances in groundwater. Because the concentration of organic carbon in groundwater is commonly less than 1 mg carbon per liter, it is difficult to isolate and characterize humic substances, and there are relatively few studies of these substances in groundwater. 

TABLE 2. Concentration of Organic Carbon and Humic Substances in Selected Groundwaters"... 

As explained in the first section of this chapter, low concentrations of humic substances occur in groundwater from the saturated zone. Thus, geochemi-... 

Dissolved organic carbon can be divided into six fractions hydrophobic acids, bases, and neutrals and hydrophilic acids, bases, and neutrals (Leenheer, 1981). Dissolved organic carbon fractionation data for a variety of freshwater systems are presented in Table 1. Humic substances are the major component of the hydrophobic acid fraction. These compounds range in concentration from 20 /ag/L (micrograms per liter) in groundwater to over... 

Before humic substances from water can be properly characterized, they need to be further concentrated and isolated from other solutes.)The problem of concentrating these substances from water has hindered their study in the past. However, in the last 20 years, methods have been developed to improve the efficiency of the concentration process. Humic substances are now easily isolated and concentrated from waters with very low concentrations of organic carbon such as groundwater (see Chapter 4). Numerous concentration methods are available (Table 4). The more commonly used methods are evaluated below. 

A scheme devised by Thurman and Malcolm (1981) uses XAD-8 to concentrate and isolate aquatic humic substances. According to this scheme, the sample is first filtered through 0.45 /am silver-membrane filters and acidified. After concentration on XAD-8 resin, the humic acid fraction is precipitated at pH 1. Both humic acid and fulvic acid fractions are hydrogen saturated by passing the sample through a cation-exchange resin in the H-form. These fractions are then freeze-dried to yield low-ash samples of aquatic humic and fulvic acids. This extraction scheme is outlined in Table 7. These authors successfully isolated humic substances from a number of surface and groundwaters. Even samples with DOC values of 0.7 mg C/L could be proc-... 

During the last 15 years, interest in the study of aquatic humic substances has increased. The problems associated with isolating and concentrating this material from aqueous solution largely have been overcome, and humic substances can be easily extracted from any aquatic sample. Humic substances have been successfully isolated from waters with very low DOC values, such as seawater and groundwater, as well as more concentrated systems. 

A seven year smdy on the groundwater in the Paris region of France, revealed DEA was present at a concentration above that of its parent compound. The atrazine degradation pathway and the higher solubility of DEA in water may explain this finding. Recent work showed that photolysis of triazines and acetanilides followed pseudo first order kinetics, and the photodegradation in soils was accelerated as the content of organic matter increased. Another study showed that humic substances enhanced terbutylazine photolysis. ... 

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