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Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 2/20/2015 Publication Date: 3/11/2015 Citation: Kennedy, C.D. 2015. Hydrologic and nutrient response of groundwater to flooding of cranberry farms in southeastern Massachusetts, USA. Journal of Hydrology. 525:441-449. Interpretive Summary: Despite close to 90% of Massachusetts growers flooding for fall harvesting and winter protection, the hydrological and water-quality effects of cranberry floods are still poorly known. Here, a detailed analysis of the harvest and winter floods shows that these unique hydrological events contribute significantly to groundwater recharge of the regional aquifer system, with measurements of groundwater recharge derived from cranberry floods exceeding current estimates by a factor of four. Moreover, the chemistry of cranberry floodwaters is low in nitrate and phosphorus, both of which have been implicated in regional surface water impairment, and results show that floodwater recharge can represent a source of fresh, clean water that reduces nitrate concentrations in nutrient-impaired unconfined aquifers. Technical Abstract: Seasonal flooding of cranberry farms is essential for long-term sustainability of cranberry production in southeastern Massachusetts, with roughly 90% of growers flooding for fall harvesting and winter protection. Although considered a significant source of recharge to the regional unconfined aquifer system, periodic flooding may chemically mobilize and physically transport nutrients from cranberry farms to groundwater. Given the paucity of information on groundwater exchange with cranberry floodwaters, hydrometric measurements were used to solve for the residual term of groundwater recharge in water budgets for three cranberry farms during the harvest and winter floods. In addition, continuous monitoring of water-table depth and discrete sampling of groundwater for analysis of nitrate, ammonium, dissolved inorganic nitrogen (DIN), and total dissolved phosphorus (TDP) were conducted to evaluate the hydrologic and nutrient response of groundwater to flooding of cranberry farms. Normalized by farm area, mean values and standard deviations of groundwater recharge were 11 (+/-6) cm and 46 (+/-12) cm for the harvest and winter floods, respectively. The factor-of-four difference in ground recharge was related to holding times that, on average, were 20 days longer for the winter flood. The total estimated recharge of 57 cm was about four times higher than that assigned to the cranberry farms in transient models of groundwater flow for the region. During the floods, the aquifer responded quickly with increases in water-table depth from 10-20 cm for wells within 10 m of the farm. For wells at greater distances, water-table depth decreased slightly or showed only minimal variation in response to flooding, suggesting a zone of influence within 100 m from the flooded edge of cranberry farms. Nutrient analysis of 50 groundwater samples collected from 10 wells indicated generally low concentrations of TDP in groundwater (less than 0.32 microM for 86% of the samples) and that nitrate accounted for 85% of the DIN concentration in groundwater. A general pattern emerged of decreasing nitrate concentration in groundwater with distance from the farm, with values of less than 3.6 microM for wells located, on average, about 100 m from the farm. For one well located at 10 m from the edge of the farm, nitrate concentrations in groundwater decreased in response to the winter flood by a factor of 5, from 566 microM (pre-flood) to 141 microM (post-flood). For this well, measured groundwater concentrations of chloride (a chemically conservative tracer) suggested that recharge from cranberry floods can represent a source of fresh, clean water that reduces groundwater concentrations of nitrate in nutrient-impaired unconfined aquifers. |
