Projects

Current Initiative

Global groundwater storage trends as measured by GRACE, from TREND magazine 2019 with Jay Famiglietti (here)

My current research seeks to establish better links between groundwater depletion measured from GRACE, land subsidence from InSAR, and numerical groundwater models.

As groundwater is pumped from an aquifer, sediment grains rearrange in response. Finer, clay-like sediments, often suspended in water before pumping, tend to lay flat as water depletes. This causes compaction of the aquifer that is often irreversible and inelastic. This causes surface deformation and land subsidence that is detectable via InSAR (interferometric synthetic aperture radar) and GPS.

PALSAR (left), Radarsat-2 (middle), and Sentinel-1 (right) maps of subsidence in the San Joaquin Valley. Produced by Zhen Liu and Tom Farr for CADWR. The colors show areas of more or less severe subsidence over the long term (left, 2007-2010) and two recent years, 2014-2015 (middle) and (right). Results show an intensification of the subsidence signal in the southern Central Valley over recent years.
PALSAR (left), Radarsat-2 (middle), and Sentinel-1 (right) maps of subsidence in the San Joaquin Valley. Produced by Zhen Liu and Tom Farr for CADWR. The colors show areas of more or less severe subsidence over the long term (left, 2007-2010) and two recent years, 2014-2015 (middle) and (right). Results show an intensification of the subsidence signal in the southern Central Valley over recent years.

With this project, we seek to better understand the linkages between geologic properties and patterns of subsidence, at depth and over time, using GRACE, InSAR, and a groundwater model. Final deliverable of the initiative will aid Groundwater Sustainability Agencies in sustainable use of groundwater across California’s Central Valley, in compliance with the 2014 Sustainable Groundwater Management Act.

Past Initiatives

IMG_2989

Beach aquifers are hosts of dynamic mixing zones between fresh groundwater and saline seawater. Seawater, driven up the beachface by waves and tides, infiltrates into the aquifer and meets the seaward-discharging fresh groundwater, creating and maintaining a reactive intertidal circulation cell. The intertidal circulation cell is highly dynamic and has been shown to respond to hydrologic, geologic, and topographic changes over various time scales (waves, tides, and seasons).

Spatial patterns of groundwater reactivity in an intertidal beach aquifer

Measured (1) and modeled (2) distributions for (a) salinity, (b) dissolved oxygen, (c) nitrate, and (d) N2 in the intertidal beach aquifer at Cape Shores, DE. The flow vectors in Figure 7a-2 show the tidally averaged flow direction and magnitude. The horizontal dotted lines in both panels are MHHW and MSL. MLLW is contiguous with the topography at the base of the beachface. The modeled 1, 10, and 20 salinity contours are shown in Figures b-2, c-2, and d-2.

Within the cell, land-derived nutrients delivered by fresh groundwater are transformed or attenuated. We investigated the relationship between physical flow and mixing processes with porewater samples and incubation experiments. Biogeochemical reactions within the circulation cell were highly related to flowpaths, with heightened oxic respiration near the infiltration zone and progressive dominance of denitrification near the discharge zone. The results of this project were published in JGR Biogeosciences and selected for an EOS Research Highlight.

Migration of beach aquifer reaction zones : Transient seasonal patterns of organic carbon and chemical reactions

Conceptual diagram of carbon movement within the intertidal beach aquifer. Groundwater flow in the current hydrologic condition (a) transports marine DOC and particulate carbon (POC) (i), which can become entrapped along the flow path as it moves through the sediments (i to ii). POC actively leaches DOC (iii). Pore water POC is sampled with porewater, representing mobile POC and/or some component of immobile POC mobilized by sampling (iv). Entrapped POC from a previous hydrologic condition (b) can appear as a hot spot (v) under current conditions.

Extensive characterization of porewater and sediments spanning two years revealed the seasonal migration of reaction zones. While oxic respiration closely followed the changes to groundwater flowpaths indicated by salinity patterns, spatial locations of denitrification was more variable. This was partially attributed to the heterogeneous distribution of reactive organic carbon within the beach, due to the filtration effect of beach sediments.

As part of this work, I modeled, for the first time, the biogeochemical impacts of particulate matter to fresh and saline water quality in beaches. This was done by coupling a variable-density groundwater flow model and a reactive transport model. An article detailing the outcomes were published in JGR Biogeosciences, with data and model separately published here.

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