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Areal, Vertical, and Temporal Differences in Ground Water Chemistry: II. Organic Constituents¹

ABSTRACT

Selected organic components in samples collected from monitoring wells and from surface water are shown to follow definite areal, vertical, and temporal trends in concentration. Atrazine (2-chloro-4-[ethylamino]-6-[isopropylamino]-s-triazine), alachlor (2-chloro-2' ,6' - diethyl-N-[methoxymethyl]acetanilide) and dieldrin (1,2,3,4,10,10-hexachloro-exo-6,7-epoxy-1,4,4a,5,6,7,8,8a-octahydro-1,4-endo, exo-5,8-dimethanonaphthalene) were measured in 64, 35, and 35 water samples, respectively. The atrazine amounts ranged from < 0.01 to 88 µg/liter. Peak concentrations were observed in shallow well water downgradient from irrigated fields at the end of the irrigation season. The areal and vertical distributions of atrazine are closely associated with those of nitrate-nitrogen (NO₃-N), which was measured as an indicator of deep percolation from irrigated croplands. However, temporal variations in atrazine concentrations suggest that it is a nonconservative constituent of ground water. Alachlor and dieldrin amounts were extremely low, being less than 0.1 µg/liter in all but one water sample which was taken from a well located in an excessively well-drained area. Dissolved organic carbon (DOC) was measured in 95 water samples and the amounts ranged from 0.2 to 4.8 mg/liter. Maximums occurred in shallow wells and there were no seasonal associations with either atrazine or NO₃-N. The DOC data suggest percolation from the unsaturated zone and partial removal from solution during vertical transport within the saturated zone.

Key Words: ground water • DOC • atrazine • alachlor • NO₃-N

¹ Contribution from the Conserv. and Survey Div., Inst. of Agric. and Nat. Resour., Univ. of Nebraska, Lincoln, NE 68588. This research was funded through a grant from the Central Platte Nat. Resour. District.

² Associate Chemist and Assistant Chemist, respectively, The Ames Lab., USDOE, Iowa State Univ., Ames, IA 50011.

³ Associate Professor, Conserv. and Survey Div., Inst. of Agric. and Nat. Resour., Univ. of Nebraska, Lincoln.

Received for publication September 24, 1979.