Based on the provided options and typical patterns observed for atrazine concentrations in groundwater, the following responses are likely correct:
B: Atrazine concentrations in groundwater were highest during July and early August. This is often due to increased agricultural runoff during the growing season when herbicides are applied.
C: Atrazine in the river water can contaminate groundwater great distances from the original source of the herbicide. This indicates the ability of atrazine to travel through the environment and affect areas far from where it was initially applied.
D: Atrazine was present in groundwater throughout the year, regardless of the distance from the river. This suggests that atrazine has persistent presence in the groundwater, indicating ongoing contamination beyond seasonal peak levels.
Note that option A contrasts with typical patterns, as groundwater contamination is often higher near river sources where agricultural runoff occurs, rather than being lower.