Is Flood Irrigation a Potential Driver of River-Groundwater Interactions and Diffuse Nitrate Pollution in Agricultural Watersheds?

Abstract

In the Po plain, northern Italy, rivers within agricultural basins display steep summer increases in nitrate (NO3−) concentrations. Flood irrigation in overfertilized, permeable soils may drive such diffuse pollution, facilitating interactions between NO3−-rich groundwater and surface waters. We discuss multiple, indirect evidence of this mechanism in the Adda, Oglio, and Mincio rivers. These rivers drain agricultural soils with elevated nitrogen (N) surpluses, averaging 139, 193, and 136 kg ha−1 in the Adda, Oglio, and Mincio watersheds, respectively. The three rivers cross a transitional area between highly permeable and impermeable soils, where summer NO3− concentrations may increase by one order of magnitude over short distances (8–20 km). Upstream of this transitional area, a major fraction of the river flow is diverted for flood irrigation, a traditional and widespread irrigation technique for permeable soils. We speculate that diverted water solubilizes soil N excess, recharges the aquifer, and transfers soil N surplus into groundwater, resulting in NO3− pollution. Groundwater–river interactions were estimated experimentally, via water and NO3− budgets in 0.3 to 1 m3 s−1 km−1 and in 1500 to 5400 kg NO3−–N day−1. The data suggest a pronounced east–west gradient of groundwater to river diffuse water inputs among the three adjacent basins, reflecting the soil permeability and the width of the river–groundwater interaction zone. Given the large stock of NO3− in groundwater, management interventions performed at the basin scale and aimed at decreasing N excess will not produce an immediate decrease in river NO3− pollution.

Spatial Variability of Nitrate Levels in Groundwater of Lokoja Town, Kogi State, Nigeria

“…Consequently, this could result in the solubilizing of excess nitrogen from the applied fertilizers, which is mobilised into groundwater reserves via infiltration of the flooded water, resulting in Nitrate contamination in groundwater observed. This has also been observed in a number of studies [11][12][13][14]. Moreover, the problem of floods in Ganaja is further exacerbated by the presence of dumps which leads to the complete subsuming of a number of dumps and septic systems (in the area) below the water table.…”

Using Macroelement Content to Characterize Surficial Water Quality of Artificial Reservoirs

“…The impact of agricultural management on surface water pollution has been addressed in many publications, e.g. (Racchetti et al, 2019;Tong & Chen, 2002). Dry and wet deposition is not insignificant for the pollution of water bodies (Luo et al, 2007).…”

Reactive Silica Traces Manure Spreading in Alluvial Aquifers Affected by Nitrate Contamination: A Case Study in a High Plain of Northern Italy

“…The former, in this area of northern Italy, are in large excess [40]. In addition, the high concentrations and the rapid decrease in nitrate in fertilized column tests suggest the quick leaching of recently applied manure when water applied to the top of the soil is abundant, as in summer (due to irrigation, e.g., [43]) or spring and autumn (due to precipitation, e.g., [70,71]). On the other hand, previous manure spreading with consequent NH 4 + loss could lead to a lower leaching of nitrate.…”

“…Moreover, several articles where soil system budgets have been calculated at the watershed level report a large and widespread nutrient surplus [36,39,40]. These articles postulate a mechanism linking fertilizer surpluses with river quality in specific river sectors, mediated by large water-demanding irrigation techniques and precipitation, favored by the permeability of soils [41][42][43].…”