After the water leaves the root zone, it percolates to the water table, mostly vertically down. From the water table it travels laterally and vertically through groundwater along groundwater flow paths. Movement in groundwater is mostly horizontal through sand and gravel layers and mostly vertical through fine-grained loam and clay layers. To understand impacts from nonpoint source pollution and to assess how soon groundwater quality may improve after actions to contol pollution are taken, it is critical to understand the time of travel from the water table to the well.
One of the core processes of CV-NPSAT efforts is the calculation of the groundwater travel times.
In the following we describe an interactive visualization of typical groundwater travel paths in the Central Valley aquifer system. Groundwater traveling into each well is represented through one hundred individual streamlines. The tool identifies the beginning position of these one hundred streamlines, at the water table. Advection and dispersion along each streamline cause water entering the well to have a wide age distribution. The age distribution at each well can be examined with this tool.
CV-NPSAT considers only average long-term groundwater flow conditions, which allows for very high computational efficiency. It does not account for day-to-day or seasonal variations in groundwater flow paths. On the other hand, CV-NPSAT implements a much more refined spatial representation of recharge, including river recharge (and groundwater discharge to rivers) across the landscape, at a scale of 150 -200 feet, using CV-SWAT information (see here to compare recharge patterns). CV-SWAT is a soil-crop-landscape model to estimate recharge and nitrate loading to groundwater, among other things (CV-SWAT was developed by the Management Practices Evaluation Program team for the Central Valley Irrigated Lands Regulatory Program in 2018). CV-NPSAT also includes a spatially highly resolved representation of well locations and the depth and length of their screens (see also Kourakos, Pauloo and Harter 2024) using CA DWR well log information. This high spatial resolution allows CV-NPSAT to properly capture the variability of flow dyanmics below the recharging landscape and around wells.
The CV-NPSAT tool's internal output is the timeline of nitrate (or other nonpoint source pollutants) at individual wells - at about 20,000 public water supply and irrigation wells and at over 55,000 domestic wells. While providing this spatially highly resolved output, the fact that CV-NPSAT represents only long-term average flow conditions, and the limitations in the representation of aquifer characteristics, water budget, and water management - historic or current - in the underlying data and development of CVHM2, C2VSIM, and CV-SWAT suggests that predictions of nitrate at individual wells are subject to some uncertainty and inaccuracy. However, due to the excellent representation of the many different ways in which recharge areas connect with wells, CV-NPSAT provides an excellent impression of the variety of source area sizes, of groundwater transport distances and of the range of groundwater ages observed across wells. Hence, when considering groups of wells, e.g., within a township, CV-NPSAT provides excellent predictions of the range of concentrations (the "histogram" or "probability distribution") observed across such a group of wells at any given time. The external output of CV-NPSAT are therefore statistical summaries of nitrate (or other pollutant) concentrations across user-defined regions in the Central Valley - from individual township, counties, GSAs, or groundwater basins to the Central Valley as a whole.
Well Explorer
The above link access the well explorer tool that is part of the CV-NPSAT web application.
