Decadal trends in the quality of groundwater used for public drinking-water supply in California, 2004–2023, California groundwater ambient monitoring and assessment program, priority basin project

U.S. Geological Survey (USGS) | February 25th, 2026

This study provides a comprehensive assessment of decadal changes in the quality of groundwater used for public drinking-water supply at 444 monitoring site

This study provides a comprehensive assessment of decadal changes in the quality of groundwater used for public drinking-water supply at 444 monitoring sites across California during 2004–2023. We assessed decadal step trends in groundwater quality for 145 water-quality constituents and geochemical indicators statewide and across geographic and land-use based network groups. We evaluated the statistical significance of directional changes (predominant increase or decrease of constituent concentrations) and the magnitude of those changes across all network groups. Uranium showed the most widespread directional and high-magnitude increases of all constituents with regulatory benchmarks statewide, particularly in the agriculture-dominated Central Valley as well as urban- and desert-dominated regions of Southern California. Fluoride and perchlorate showed the most widespread directional and high-magnitude decreases of all constituents with regulatory benchmarks statewide, which were also most pronounced in Southern California. Although arsenic and nitrate did not often register significant directional changes across network groups, they showed widespread, high-magnitude changes in both directions (increase and decrease) at levels often exceeding 10 percent of respective regulatory benchmarks statewide. Triazine herbicides (atrazine and simazine) and the gasoline oxygenate methyl tert-butyl ether (MTBE) showed significant directional decreases statewide, but not at levels considered to be of high magnitude compared to respective regulatory benchmarks. We observed significant directional and high-magnitude increases of total dissolved solids (TDS) statewide, which were most pronounced in agricultural areas. Analysis of explanatory geochemical indicators indicated that prevalent statewide increases of alkalinity and calcium were the predominant components of the observed statewide increases in TDS by mass. Widespread increases in groundwater alkalinity and calcium across agricultural and urban areas may be related, in part, to warm-season irrigation and other anthropogenic factors that have shifted soil weathering dynamics over the long term. Increasing alkalinity concentrations were related to increasing uranium concentrations, particularly in areas with aquifer materials derived from granitic rocks. Conversely, increasing calcium concentrations were related to decreasing fluoride concentrations, particularly in areas where fluoride occurred naturally at elevated concentrations. Decrease of perchlorate, triazine herbicides, and MTBE are likely related to decreased anthropogenic source inputs over time and natural attenuation in aquifers.

Global Warming Has Accelerated Significantly

American Geophysical Union (AGU) | March 6th, 2026

The rise in global temperature has been widely considered to be quite steady for several decades since the 1970s. Recently, however, scientists have started to debate whe

The rise in global temperature has been widely considered to be quite steady for several decades since the 1970s. Recently, however, scientists have started to debate whether global warming has accelerated since then. It is difficult to be sure of that because of natural fluctuations in the warming rate, and so far no statistical significance (meaning 95% certainty) of an acceleration (increase in warming rate) has been demonstrated. In this study we subtract the estimated influence of El Niño events, volcanic eruptions and solar variations from the data, which makes the global temperature curve less variable, and it then shows a statistically significant acceleration of global warming since about the year 2015. Warming proceeding faster is not unexpected by climate models, but it is a cause of concern and shows how insufficient the efforts to slow and eventually stop global warming under the Paris Climate Accord have so far been.

California's Rivers and Streams

Public Policy Institute of California (PPIC) | January 26th, 2026

Water Quality in California

Public Policy Institute of California (PPIC) | February 23rd, 2026

Spatial analysis of residential location at birth, PFAS in public water, and childhood cancers in Southern California (2000–2019)

Nature Portfolio (Springer Nature) | March 5th, 2026

Locations with significant increased risk of all cancers, leukemias, lymphomas, and other solid tumors were collocated with PFOS/PFOA contaminated water supplies. Referen

Locations with significant increased risk of all cancers, leukemias, lymphomas, and other solid tumors were collocated with PFOS/PFOA contaminated water supplies. Referent ORs for location were attenuated after adjusting for PFOS detection and in stratified analyses for children of US-born mothers but not for Mexico-born mothers. PFOS detection was associated with higher odds of neuroblastoma (OR = 1.51, 95% CI: 1.07, 2.13) and retinoblastoma (OR = 1.83, 95% CI: 1.19, 2.79), whereas all cancers (OR = 1.07; 95% CI: 0.97, 1.18), brain tumors (OR = 1.15, 95% CI: 0.96, 1.37), and other solid tumors (OR = 1.12, 95% CI: 0.98, 1.29) had non-significant associations. PFOA detection was associated with elevated risk of retinoblastoma (OR = 1.85, 95% CI: 1.14, 3.01).

Revisiting chlorophyll a thresholds for San Francisco Bay: Insights from observations of phytoplankton molecular abundance

Elsevier | February 14th, 2026

Harmful Algal Blooms (HABs) are a hazard for coastal environments worldwide; identifying screening thresholds of chlorophyll-a (chl-a) associated with inc

Harmful Algal Blooms (HABs) are a hazard for coastal environments worldwide; identifying screening thresholds of chlorophyll-a (chl-a) associated with increased risk of HABs is a management priority. Molecular surveillance of coastal phytoplankton and bivalve biotoxins could be used to link chl-a with HAB risk, but requires an understanding of whether the HAB risks increase uniformly as chl-a rises, or whether some taxa are disproportionately favored, and if these relationships vary by season. In this study, we present a novel use of molecular abundance data to investigate the scientific bases for estuarine chl-a thresholds protective against HABs. In San Francisco Bay (SFB), California, the relationship between molecular relative abundance (as measured by 18S metabarcoding) of nine different HAB taxa, absolute quantitative polymerase chain reaction (qPCR) abundance, and mussel toxin concentrations of a subset of the taxa were investigated for thresholds as a function of increasing chl-a. Our results show most HAB taxa did not increase in absolute or relative abundance during SFB’s spring bloom interval, when chl-a levels were highest (>10 µg/L) but the assemblage was dominated by non-harmful diatoms. However, several flagellated, mixotrophic taxa did increase above their molecular baseline in fall, and the combined probability of any HAB occurring above baseline was elevated when chl-a reached ∼4.6 µg/L in the fall. This work demonstrates the promise of molecular approaches in disentangling the seasonally complex interplay between stressors and phytoplankton/HAB community responses and has the potential to provide clearer, more cost-effective monitoring and mitigation strategies for managers.

2020 Urban Water Management Plan

Metropolitan Water District of Southern California (MWD) | June 7th, 2021

Metropolitan’s 2020 Urban Water Management Plan (UWMP) has been prepared in compliance with the California Water Code (CWC). This Executive Summary satisfies the requir

Metropolitan’s 2020 Urban Water Management Plan (UWMP) has been prepared in compliance with the California Water Code (CWC). This Executive Summary satisfies the requirement of CWC Section 10630.5 to include a simple lay description of information necessary to provide a general understanding of the plan, including a description of Metropolitan’s reliable water, as well as its needs, strategies, and potential challenges for the foreseeable future. This plan provides an assessment of Metropolitan’s water service reliability, describes and evaluates sources of water supply, efficient uses of water, demand management measures, implementation strategy and schedule, and other relevant information and programs. In addition to the water reliability assessments, the plan includes an evaluation of frequent and severe periods of droughts, as described in the Drought Risk Assessment, and the preparation and adoption of the Water Shortage Contingency Plan (WSCP).

Urban Water Management Plan Guidebook

California Department of Water Resources (DWR) | January 30th, 2026

In 1983, the State of California Legislature (Legislature) enacted the Urban Water Management Planning Act (UWMP Act). The UWMP Act requires an urban water supplier (Supp

In 1983, the State of California Legislature (Legislature) enacted the Urban Water Management Planning Act (UWMP Act). The UWMP Act requires an urban water supplier (Supplier) providing water for municipal purposes to more than 3,000 customers or serving more than 3,000 acre-feet annually to adopt an Urban Water Management Plan (UWMP) every five years, demonstrating water supply reliability in normal, single dry, and multiple dry water years. The UWMP Act also requires the California Department of Water Resources (DWR) to report to the Legislature on the status of UWMPs. Appendix A contains copies of the UWMP Act and other sections of the California Water Code relevant to UWMPs. Appendix B shows revisions to the UWMP Act and to Water Code. There have been no requirement changes since 2020; only definitions have been added or updated. 

Considerations and Pathways for Potential State Oversight of Community Water Systems’ Wildfire Fighting Efforts in California

University of California, Los Angeles (UCLA) | March 3rd, 2026

Since the Los Angeles Fires of January 2025, dramatic shifts have occurred in perceptions, expectations, and understanding of water systems’ role in fighting wildfires

Since the Los Angeles Fires of January 2025, dramatic shifts have occurred in perceptions, expectations, and understanding of water systems’ role in fighting wildfires in California. As a consequence, policy attention has increased regarding the potential for enhanced statewide oversight to enhance wildfire-fighting capacity across community water systems. No state in the U.S. has previously pursued this path. Water systems face many obstacles to fighting wildfires that they have not been expected or supported to overcome. At the same time, as wildfires become more urban and increasingly encroach into the service territories of community water systems, the concept introduces interesting questions and potential innovations in design and implementation that may be good investments as part of long-term climate change adaptation.

Delta-Mendota Subbasin Staff Assessment

California State Water Resources Control Board (SWRCB) | March 2nd, 2026

The Delta-Mendota Subbasin (subbasin) is located in the western portion of the San Joaquin Valley, in portions of Fresno, Madera, Merced, San Joaquin, San Benito, and Sta

The Delta-Mendota Subbasin (subbasin) is located in the western portion of the San Joaquin Valley, in portions of Fresno, Madera, Merced, San Joaquin, San Benito, and Stanislaus counties, and contains the cities of Dos Palos, Firebaugh, Gustine, Los Banos, Mendota, Newman, and Patterson. It is critically overdrafted and managed by 7 GSA Groups, comprised of 23 GSAs, which include: the Aliso Water District GSA Group, the Farmers Water District GSA Group, the Fresno County Management Areas A and B GSA Group, the Grassland GSA Group, the Northern Delta-Mendota GSA Group, the Central Delta-Mendota GSA Group, and the San Joaquin River Exchange Contractors GSA Group. During most years, agriculture accounts for more than 85% of groundwater use in the subbasin (Water Year 2019–Water Year 2024 Annual Reports). The state intervention process for the Delta-Mendota Subbasin was initiated in March 2023 when DWR determined the subbasin’s six 2022 GSPs were inadequate.  The GSAs released a single revised draft GSP in May 2024, which they adopted with slight revisions in July 2024 (2024 GSP). State Water Board staff evaluated the 2024 GSP to determine if identified deficiencies were resolved. The GSAs made significant progress. State Water Board staff concludes the GSAs amended the GSP such that a probationary designation of the Delta-Mendota Subbasin is unnecessary and recommends the Board return the subbasin to DWR’s jurisdiction for continued evaluation of local management under SGMA.