对爱荷华州地表水和地下水的长期水质研究表明，地表水和浅井中的硝酸盐浓度增加。此外，变异模式也发生了明显变化。过去，最高硝酸盐值与春季高流量相关，而最低硝酸盐值与秋季和冬季低流量相关。近年来，在秋季和冬季流量相对较低的时期，硝酸盐含量普遍较高。这些数据表明，目前可能有大量硝酸盐从排水砖或浅层含水层进入地表水。在农田中施用氮肥似乎是供水中硝酸盐的主要来源，地表水和地下水中硝酸盐浓度的未来趋势将取决于农业实践和使用的肥料。 包括4个参考文献、表格和图表。

Long-term water quality studies of surface waters and groundwaters in Iowa have shown increases in nitrate concentrations in surface supplies and shallow wells. In addition, the patterns of variations changed distinctly. In the past, highest nitrate values correlated with high spring flow, whereas minimum values occurred in conjunction with low flow in the fall and winter. During recent years, high nitrate values were common during periods of relatively low flow in the fall and winter. These data indicate that substantial quantities of nitrate currently may be entering surface waters from drain tiles or superficial aquifers. The application of nitrogenous fertilizers to agricultural land appears to be the primary source of nitrates in water supplies and future trends of nitrate concentrations in surface waters and groundwaters will depend on farming practices and fertilizers used. Includes 4 references, tables, figures.