Santa Cruz Mid‐County Basin Groundwater Management Biennial Review and Report Water Years 2015‐2016

Santa Cruz Mid‐County Groundwater Agency | July 18th, 2017

Long‐term overdraft of the basin has led to ongoing risk of seawater intrusion. As a result of long‐term recovery and an acceleration of recovery over the las

Long‐term overdraft of the basin has led to ongoing risk of seawater intrusion. As a result of long‐term recovery and an acceleration of recovery over the last two years, average groundwater levels in Water Year 2016 met the established protective elevations at the most monitoring wells since the wells were installed. Average groundwater levels in Water Year 2016 met established protective elevations at 8 of 13 of the wells. Since five wells have average groundwater levels below established protective elevations, full basin recovery has not been achieved and the basin is still considered in long‐term overdraft. (Figure ES‐ 1). Recovery of the basin and overdraft will be eliminated when coastal groundwater levels rise to protective elevations at all coastal wells. The MGA will also need to reevaluate the established protective elevations for the GSP.

Science, society, and the coastal groundwater squeeze

American Geophysical Union (AGU) | April 28th, 2017

Coastal zones encompass the complex interface between land and sea. Understanding how water and solutes move within and across this interface is essential for managing r

Coastal zones encompass the complex interface between land and sea. Understanding how water and solutes move within and across this interface is essential for managing resources for society. The increasingly dense human occupation of coastal zones disrupts natural groundwater flow patterns and degrades freshwater resources by both overuse and pollution. This pressure results in a ‘‘coastal groundwater squeeze,’’ where the thin veneers of potable freshwater are threatened by contaminant sources at the land surface and saline groundwater at depth. Scientific advances in the field of coastal hydrogeology have enabled responsible management of water resources and protection of important ecosystems. To address the problems of the future, we must continue to make scientific advances, and groundwater hydrology needs to be firmly embedded in integrated coastal zone management. This will require interdisciplinary scientific collaboration, open communication between scientists and the public, and strong partnerships with policymakers.

Screening of Pharmaceuticals in San Francisco Bay Wastewater

San Francisco Estuary Institute (SFEI) | October 1st, 2018

Previous studies have shown that pharmaceuticals are widely detected in San Francisco Bay, and some compounds occasionally approach levels of concern for wildlife. In 20

Previous studies have shown that pharmaceuticals are widely detected in San Francisco Bay, and some compounds occasionally approach levels of concern for wildlife. In 2016 and 2017, seven wastewater treatment facilities located throughout the Bay Area voluntarily collected wastewater samples and funded analyses for 104 pharmaceutical compounds. This dataset represents the most comprehensive analysis of pharmaceuticals in wastewater to date in this region. On behalf of the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP), the complete dataset was reviewed utilizing RMP quality assurance methods. An analysis of influent and effluent information is summarized in this report, and is intended to inform future monitoring recommendations for the Bay.

Scrutinizing the Delta Cross Channel

CALFED | June 1st, 2001

Sea-level rise impacts on seawater intrusion in coastal aquifers: Review and integration

Journal of Hydrology (Elsevier) | February 6th, 2016

Sea-level rise (SLR) influences groundwater hydraulics and in particular seawater intrusion (SWI) in many coastal aquifers. The quantification of the combined and relativ

Sea-level rise (SLR) influences groundwater hydraulics and in particular seawater intrusion (SWI) in many coastal aquifers. The quantification of the combined and relative impacts of influential factors on SWI has not previously been considered in coastal aquifers. In the present study, a systematic review of the available literature on this topic is first provided. Then, the potential remaining challenges are scrutinized. Open questions on the effects of more realistic complexities such as gradual SLR, parameter uncertainties, and the associated influences in decision-making models are issues requiring further investigation. We assess and quantify the seawater toe location under the impacts of SLR in combination with recharge rate variations, land-surface inundation (LSI) due to SLR, aquifer bed slope variation, and changing landward boundary conditions (LWBCs). This is the first study to include all of these factors in a single analysis framework. Both analytical and numerical models are used for these sensitivity assessments. It is demonstrated that (1) LSI caused by SLR has a significant incremental impact on the seawater toe location, especially in the flatter coasts and the flux-controlled (FC) LWBCs, however this impact is less than the reported orders of magnitude differences which were estimated using only analytical solutions; (2) LWBCs significantly influence the SLR impacts under almost all conditions considered in this study; (3) The main controlling factors of seawater toe location are the magnitudes of fresh groundwater discharge to sea and recharge rate. Regional freshwater flux entering from the landward boundary and the groundwater hydraulic gradient are the major contributors of fresh groundwater discharge to sea for both FC and head-controlled (HC) systems, respectively; (4) A larger response of the aquifer and larger seawater toe location changes are demonstrable for a larger ratio of the aquifer thickness to the aquifer length particularly in the HC systems; (5) The lowest sensitivity of seawater toe location is found for the density difference ratio of the seawater and freshwater, and also for the aquifer bed slope; (6) The early-time observations show seawater fingers below the inundated lands due to SLR which are diminished and ultimately extinguished; and (7) A less than 2% reversal effect on the seawater toe location after overshoot mechanism is observed in the transient simulations which suggests that this mechanism is an insignificant and impractical factor compared to other more significant factors.

Seasonal and Long-Term Clarity Trend Assessment of Lake Tahoe, California–Nevada

U.S. Geological Survey (USGS) | July 20th, 2022

The clarity of Lake Tahoe, observed using a Secchi disk on a regular basis since the late 1960s, continues to be a sentinel metric of lake health. Water clarity is influe

The clarity of Lake Tahoe, observed using a Secchi disk on a regular basis since the late 1960s, continues to be a sentinel metric of lake health. Water clarity is influenced by physical and biological processes and has declined in the five decades of monitoring, revealing differences between summer (June–September) and winter (December–March). This document summarizes key findings of a study of Lake Tahoe water clarity, including long-term variability and the relative importance of several influencing variables and processes. This study, prepared in cooperation with the Nevada Division of Environmental Protection, focused on (1) an apparent divergence in clarity trends between summer and winter periods, (2) observed changes in in-lake physical and ecological variables that may influence or control seasonal and annual clarity trends, and (3) five research hypotheses regarding lake clarity that were developed by Lake Tahoe management agencies. Previously collected data were used to complete this study. Trend analysis confirmed that winter clarity stabilized (that is, there is no longer a statistically significant trend up or down) during the last 20 years. Evaluation of clarity for selected months in the 50-year Secchi disk clarity dataset showed that only two summer months, July and August, had statistically significant decreases in clarity from 2000–19. Different subsets of available data were analyzed to reveal the presence or absences of trends for each season, decade, and month. Five hypotheses related to lake clarity were part of the study described by this report. Hypothesis 1 stated that clarity is controlled predominantly by the distribution and volumetric density of fine particles in suspension. This hypothesis was studied using available data describing in-lake fine (0–20 micrometers) particles from 2008–19. Water clarity was negatively correlated with in-lake particle abundance, with particles in the 1.0-4.6 μm range having the greatest effect, consistent with light-scattering theory. Estimated abundances of diatoms of the genus Cyclotella also were found to be negatively correlated with clarity. Data limitations precluded a complete investigation of hypothesis 2, which stated that the observed improvements in winter water clarity are a response to decreasing fine suspended-sediment concentrations in the lake resulting from load reductions from upland sources in and near urbanized areas. Data describing fine-sediment loading from urban areas to the lake were only available since 2014, and only once or twice per month. A slight, statistically significant, negative correlation was identified between urban fine-particle loading and monthly lake clarity with a 4-month lag. Particle abundance in monitored streams is highly correlated with simultaneous particle abundance in the lake.

Seasonality of Retreat Rate of a Wave-Exposed Marsh Edge

American Geophysical Union (AGU) | July 10th, 2024

Waves can cause destruction of marshlands when they collide with the boundary of the marsh area. We studied how the speed of marsh loss from waves can change between seas

Waves can cause destruction of marshlands when they collide with the boundary of the marsh area. We studied how the speed of marsh loss from waves can change between seasons with different climates. To do this we made five 3D models of a marsh in South San Francisco Bay over a single year and looked at their differences between summer/fall/winter/spring. We found fast marsh loss in the seasons where daily winds are common (spring/summer), and minor changes in seasons with storms but without daily winds (fall/winter). Across our entire marsh, we saw lots of variety in how much it was changing, including some addition (i.e., positive change) to the mudflats near the marsh in the fall/winter. When we compared the marsh loss over one year to what was lost between 2004 and 2019, the speeds were similar, so we believe our study period was similar to long‐term marsh dynamics. This work encourages incorporating different seasonal climates into expectations of how marshes evolve, and supports that we need to think about marshes and mudflats as connected in how they change over time.

Seawater intrusion processes, investigation and management: Recent advances and future challenges

Advances in Water Resources (Elsevier) | January 17th, 2013

Seawater intrusion (SI) is a global issue, exacerbated by increasing demands for freshwater in coastal zones and predisposed to the influences of rising sea levels and ch

Seawater intrusion (SI) is a global issue, exacerbated by increasing demands for freshwater in coastal zones and predisposed to the influences of rising sea levels and changing climates. This review presents the state of knowledge in SI research, compares classes of methods for assessing and managing SI, and suggests areas for future research. We subdivide SI research into categories relating to processes, measurement, prediction and management. Considerable research effort spanning more than 50 years has provided an extensive array of field, laboratory and computer-based techniques for SI investigation. Despite this, knowledge gaps exist in SI process understanding, in particular associated with transient SI processes and timeframes, and the characterization and prediction of freshwater–saltwater interfaces over regional scales and in highly heterogeneous and dynamic settings. Multidisciplinary research is warranted to evaluate interactions between SI and submarine groundwater discharge, ecosystem health and unsaturated zone processes. Recent advances in numerical simulation, calibration and optimization techniques require rigorous field-scale application to contemporary issues of climate change, sea-level rise, and socioeconomic and ecological factors that are inseparable elements of SI management. The number of well-characterized examples of SI is small, and this has impeded understanding of field-scale processes, such as those controlling mixing zones, saltwater upconing, heterogeneity effects and other factors. Current SI process understanding is based mainly on numerical simulation and laboratory sand-tank experimentation to unravel the combined effects of tides, surface water–groundwater interaction, heterogeneity, pumping and density contrasts. The research effort would benefit from intensive measurement campaigns to delineate accurately interfaces and their movement in response to real-world coastal aquifer stresses, encompassing a range of geological and hydrological settings.

Sediment Management (Resource Management Strategy)

California Department of Water Resources (DWR) | July 29th, 2016

Sediment in California is a valuable resource when it is properly managed, which results in multiple water benefits, environmental health, economic stability, and coastal

Sediment in California is a valuable resource when it is properly managed, which results in multiple water benefits, environmental health, economic stability, and coastal safety. Sediment definitions vary among the professional disciplines. Sediment, as reflected in this resource management strategy report, is composed of natural materials and used contextually as follows: 1. Geology considers sediment to be the solid fragmented material, such as silt, sand, gravel, chemical precipitates, and fossil fragments, which has been transported and deposited by water, ice, or wind, or that accumulates through chemical precipitation or secretion by organisms, and that forms layers on the Earth’s surface. Sedimentary rocks consist of consolidated sediment. 2. The U.S. Environmental Protection Agency (EPA) and U.S. Army Corps of Engineers (USACE) regard sediment as material, such as sand, silt, or clay, suspended in or settled on the bottom of a water body. Sediments can come from anywhere and be just about anything. Organic and inorganic material alike can become bits of matter tiny enough to be picked up and carried along with a moving fluid. Organic sediments are made up of mostly plant and animal debris. Inorganic sediments are divided into two main groups — coarse-grained sediments and fine-grained sediments. Coarse- grained sediments are boulders, cobbles, gravel, and sand. Fine-grained sediments are silts and clays. Sediment deposits, like tree rings, can serve as a record of natural history. A further important distinction is whether they are clean sediments or contaminated sediments, as this greatly affects the manner in which they can be used as beneficial material or if they must be isolated from their surrounding environment.

Sediment Supply to San Francisco Bay, Water Years 1995 through 2016: Data, trends, and monitoring recommendations to support decisions about water quality, tidal wetlands, and resilience to sea level rise

San Francisco Estuary Institute (SFEI) | June 11th, 2018

Knowledge of the status and trends of sediment supply to San Francisco Bay is critically important for management decisions about dredging, marsh restoration, flood contr

Knowledge of the status and trends of sediment supply to San Francisco Bay is critically important for management decisions about dredging, marsh restoration, flood control, contaminants, water clarity (in relation to primary production), and sea level rise. Several site-specific studies of sediment supply to San Francisco Bay have been conducted, but no synthesis of recent studies is available. The purpose of this report is to synthesize the best available data and knowledge to answer a few of the key study questions related to sediment supply to the Bay.  This synthesis report was prepared jointly by the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) and the U. S. Geological Survey (USGS) with funding from both organizations. The project is meant to be a step in the development of a more comprehensive sediment management and monitoring strategy for the Bay.