New insights into the controlling factors of nitrate spatiotemporal characteristics in groundwater of Dagu aquifer in Qingdao, China

Identifying spatiotemporal variation of groundwater NO3-N and its primary controlling factors are vital for groundwater protection. This study, under the data scarce conditions and based on time series monitoring data in Dagu aquifer, applied methods including hydrochemical ion ratio, multiple linear regression, support vector regression and grey relational analysis and dedicated to revealing primary controlling factors of temporal variation patterns of groundwater NO3-N. The results showed that agricultural and manure fertilizer are the main sources of NO3-N in north and central area (vegetable farming area), and that domestic sewage discharge and manure fertilizer are the main sources of NO3-N in south area (residential and grain planting area). In addition, results identified the dominant influencing factors of variation of NO3-N in different regions, with human wastewater discharge, nitrogen load amount and water-table depth being the dominant factors of variations of NO3-N in north area, human wastewater discharge being the main factor of variations of NO3-N in central area, and irrigation water and human wastewater being the leading factors of variations of NO3-N in south area. Moreover, types of controlling factors can influence the seasonal variations of NO3-N. NO3-N in vegetable farming area that dominantly affected by fertilization generally shows higher concentration and larger variation range of concentration during summer and autumn than that during spring. NO3-N which mainly affected by human wastewater discharge and manure inputs shows minimal seasonal variation of mean concentration. NO3-N in grain area influenced by irrigation could show more significant variations during spring and autumn than that during summer. The conclusions can enhance understandings of major influencing factors on NO3-N variation in local aquifer. Importantly, the dominant roles of water-table depth and irrigation in NO3-N variation of N2 site (vegetable planting area) and S5 site (grain planting area), respectively, were highlighted. [Display omitted] •Major influence factors of nitrate's temporal variation were revealed under data scarce conditions.•Water-table and irrigation took important roles in temporal variation of groundwater nitrate.•Seasonal variation features of nitrate concentration in different area were concluded.•Multi statistical methods reasonably identified the major influence factors of nitrate's variation.