Hydrology and Water Quality CEQA Mitigation Measures

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The California Environmental Quality Act (CEQA Overview) requires that state and local agencies consider and disclose the potential environmental impacts of a proposed project. CEQA states that projects should not be approved as proposed if there are feasible alternatives or feasible mitigation measures available which would substantially lessen the significant environmental effects of such projects (PRC § 21002).

Part of the CEQA process involves determining the type of environmental impacts likely to result. For hydrology and water quality, the CEQA Handbook defines impacts as the need to assess whether the project will affect surface and groundwater resources (including aquifer characteristics and water quality), whether runoff will be generated, that could affect flooding or drainage characteristics, or whether flooding from storm events or dam failure inundation might result.

The hydrology and water quality section of an environmental impact report should also include information on the hydrological characteristics of the site prior to any proposed project. These include existing water bodies, drainage patterns, flood plains, existing drainage and/or flood control facilities, groundwater recharge potential and surface and groundwater quality.

The availability of water supplies and infrastructure to serve project demand are addressed in the Services and Utilities and Energy section of the CEQA Handbook.

Information about mitigation approaches to these impacts was taken from a sampling of EIR and MND documents as these types of CEQA documents generally require mitigation. For a more complete list of CEQA documents, see CEQA Overview.

Standards of Significance

Standards of significance must be determined for the following potential hydrology and water quality environmental impacts to determine whether the proposed project:

  • Violates any water quality standards or waste discharge requirements;
  • Substantially depletes groundwater supplies or interfere substantially with groundwater recharge such that there would be a net deficit in aquifer volume or a lowering of the local groundwater table level (i.e., the production rate of pre-existing nearby wells would drop to a level which would not support existing land uses or planned uses for which permits have been granted);
  • Substantially alters the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner which would result in substantial erosion or siltation on- or off-site;
  • Creates or contributes runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff;
  • Otherwise substantially degrades water quality;
  • Places housing within a 100-year flood hazard area, as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map;
  • Places structures within a 100-year flood hazard area that would impede or redirect flood flows;
  • Exposes people or structures to a significant risk of loss, injury or death involving flooding, including flooding as a result of the failure of a levee or dam; or
  • Inundations by seiche, tsunami, or mudflow [1]


Detention Basin image by bluegreenbldg.org

Summary of Current Groundwater Supplies

The Monterey Bay Area is unusual in that most farms and municipalities rely primarily on groundwater and more water is pumped out than recharged to the aquifer on an average yearly basis. When this overdraft condition occurs in wells near the coast, it allows for seawater intrusion. The Pajaro Valley and Salinas Basin aquifers have been in a state of overdraft for many years and have sea water intrusion in the coastal areas. In both cases, increased agricultural production and urban development have accelerated the rate of intrusion[2]. Improvements to Monterey Peninsula's water supply system are needed to provide a buffer against the impacts of dry years in which shortages rapidly develop due to inadequate storage on the Carmel River and increased pumping and overdraft of ground water basins[3]. Water agencies in these areas are seeking to combat the problems of groundwater supply and seawater intrusion through increased conservation, aquifer recharge projects, like the Salinas Valley Water Project (SVWP), and the use of recycled water for irrigation.

Groundwater Impact Mitigation Plans

Under CEQA, proposed projects must outline mitigation measures designed to offset impacts to groundwater quality and supply. A review of several Mitigated Negative Declarations (MND) and Environmental Impact Reports (EIR) from proposals currently under review yielded the following plans for lessening the impacts to groundwater likely to result from new development.

Type Project Level of Significance Mitigation Plan
EIR Ferrini Ranch - 870 acres / 146 houses Less than significant The property owner contributes financially towards the SVWP, so the proposed project is considered to have a long-term sustainable groundwater supply.
EIR Monterey Bay Shores Resort- 39 acres / 341 units Less impact than approved project The Monterey County Superior Court ruled in 2006 that the property owner is entitled to 149 acre feet of water per year regardless of constraints on groundwater usage that have been imposed on California American Water and others.
EIR September Ranch - 891 acres / 94 houses Less than significant No mitigation measures required. The aquifer underlying the proposed development will provide enough water to meet the estimated needs of future residents while producing no significant effect on the adjacent Carmel Valley aquifer due to limited hydrologic connectivity.
EIR Rancho Canada - 72 acres / 280 units Less than significant Post-project, on-site aquifer recharge facilities will provide for 33.2 acre feet of recharge, which is roughly equal to the current recharge rate of 34.9 acre feet.
MND Annexation of Oak Ridge & Via Del Sol Subdivisions No impact Water supplied to the proposed annexation and sphere of influence amendment areas would be from existing Aromas Water District wells that have already been permitted and developed.
MND Salinas Valley State Prison - medical offices No impact Monterey County Water Resources Agency (MCWRA) in recalculating SVSP’s benefit assessment for the Salinas Valley Water Project (SVWP) to reflect the projected two percent increase in additional water demand.

Drainage and Runoff

Summary of Regulatory Requirements Concerning Drainage and Runoff

Building codes for new construction and redevelopment require that runoff rate be minimized to predevelopment conditions where new impervious surfaces would reduce localized infiltration area and capability or increase the volume and rate of stormwater runoff (SCC). In some cases, it may be required to detain increases in surface runoff and design for the difference between a 10-year pre-development storm event and a 100-year post-development storm event [4]. Drainage improvements for runoff from impervious surfaces are required to be engineered to minimize erosion through the use of rocked culvert inlets and outfalls, energy reducers, and location of culverts. Design features include reseeding exposed slopes as well as minimizing the use of artificial slopes.

Drainage Impact Mitigation Plans

Under CEQA, proposed projects must outline mitigation measures designed to offset impacts to drainage systems. A review of several Mitigated Negative Declarations (MND) and Environmental Impact Reports (EIR) from proposals currently under review yielded the following plans for lessening the impacts to drainage systems likely to result from new development.

Type Project Level of Significance Mitigation Plan
EIR Harper Canyon Subdivision Less than significant with mitigation Mitigation will include detention ponds to limit stormwater runoff generated by the development. The ponds will be designed to contain the difference between the 100-year post-development runoff rate and the 10-year pre-development runoff rate. Detention basins will be fenced for public safety.
EIR Ferrini Ranch Subdivision Less than significant impact A majority of stormwater runoff generated by the project would be collected onsite via a stormwater drainage system installed within the right of way of proposed roadways to convey stormwater to detention basins throughout the property.
EIR September Ranch Less than significant with mitigation Increased runoff and earlier storm peaks as a result of the project will be mitigated using a series of detention basins throughout the project area designed to meet the additional runoff created during a 100-year storm event.
MND Carmel Stone Mine Less than significant with mitigation Runoff discharge from the mine will be retained onsite using two detention ponds with a capacity of approximately 600,000 gallons each. A spillway/culvert overflow will be installed for discharge into natural historic drainage corridors.
MND The Gateway Center No impact with mitigation Stormwater runoff from the site will be directed through grassed bioswales/biofilters to a detention/retention basin designed to retain runoff and percolate it back to groundwater so that the amount of runoff from the site will not exceed the rate or volume under existing conditions.
MND The Promontory at CSUMB No impact The project includes a lot-line adjustment to secure 7,992 square feet of additional property to accommodate 0.55 acre feet per year of stormwater runoff from the project for the construction of an infiltration basin. With this basin the project would not exceed the capacity of the stormwater drainage system.
MND Annexation of Oak Ridge & Via Del Sol Subdivisions No impact The proposed project elements would not result in new development or impervious surfacing, except for a potential minor increase with construction of a new water storage tank. The project would result in no impact regarding alteration of drainage patterns and watercourses and potential subsequent erosion.


  1. Hydrology and Water Quality Standards of Significance [1]
  2. Pacific Institute [2]
  3. Central Coast Regional Water Quality Control Board [3]
  4. Monterey County Coast Subdivision Ordinance [4]



This page may contain student work completed as part of assigned coursework. It may not be accurate. It does not necessary reflect the opinion or policy of CSUMB, its staff, or students.