Which of the following is a major water quality issue associated with agricultural practices?

Nutrient runoff and pesticide contamination is a significant concern linked to agricultural practices due to the heavy use of fertilizers and pesticides in farming. When it rains or during irrigation, these substances can wash off the fields and enter nearby waterways. This runoff can lead to various water quality issues, such as eutrophication, where excess nutrients (especially nitrogen and phosphorus) stimulate algal blooms in rivers and lakes. These algal blooms can deplete oxygen levels in the water, harming aquatic life and disrupting ecosystems. Additionally, pesticides in runoff can contaminate drinking water sources, posing risks to human health and further impacting biodiversity in local environments.

In contrast, options like increased forest cover or decreased soil erosion generally imply positive environmental outcomes rather than issues, while excessive carbon dioxide absorption is more related to climate change than direct water quality problems associated with agriculture.