NHIS: Non-Hazardous Hydrocarbon Impacted Soil Program
wedge welding machine

A wedge welding machine joins the edges of two sections of liner material by melting the two surfaces together. This joined seam creates an air channel which is pressure tested to ensure their are no gaps in the joined seam.


The City of Santa Maria owns and operates the Santa Maria Regional Landfill, which serves residents and businesses throughout the Santa Maria Valley. The Santa Maria Regional Landfill has three main sections used for waste disposal:

  • The unlined Inactive Area (used prior 1970s). This was primarily used as a burn dump from the 1950s until the early 1960s, at which time open burning of waste was outlawed.
  • The unlined Closed Active Area (used until 2002). Prior to the promulgation of regulations requiring liners, the unlined Closed Active Area was used for waste disposal operations by burying the unburned waste materials.
  • The industry-standard lined Active Area (currently used). This uses a double-lined disposal cell equipped with a leachate collection and removal system. Landfill leachate is the residual liquid that percolates through the decomposing trash.

In 1996 the City faced tremendous challenges when the landfill was issued a Cleanup or Abatement Order (CAO No. 96-27) from the Central Coast Regional Water Quality Control Board (CCRWQCB). The Order confirmed that contaminants had reached the groundwater from liquids passing through decomposing trash. The CCRWQCB threatened to close down landfill activities unless the City took swift action.

Specific corrective actions included:

  1. Installing a gas collection system that would help remove volatile organic compounds and prevent them from becoming part of leachate.
  2. Placing an interim soil closure cover in the unlined “Inactive Area” to ensure that rain water would migrate to the drainage system instead of ponding and percolating into the buried waste, thereby generating leachate.
  3. Creating a comprehensive water quality monitoring program to ensure groundwater protection.
  4. Constructing a double-lined cell equipped with a leachate collection and management system.
  5. Mining soil at the adjacent riverbed to ensure sufficient levee capacity from infiltrating the buried waste.
  6. Adding the household hazardous waste collection facility to eliminate potential hazardous wastes from being buried and contributing to leachate contamination.
  7. Improving landfill operations to minimize contact runoff.

These corrective actions brought the groundwater contamination levels back down to safe levels.

The decision was made to close the existing unlined Closed Active Area to prevent further leachate generation contamination. In May 2001, the CCRWQCB issued landfill Waste Discharge Requirements (WDR No. 01-041), which required the landfill to discontinue disposal operations at the unlined Closed Active Area by November 2002, but which was contingent on completing construction of a regulation-compliant, double-lined disposal cell (lined Active Area).

The double-lined disposal cell (part of the Active Area) was completed and used for taking refuse material in November 2002, while waste disposal operations in the unlined Closed Active Area were stopped.

The double-lined Active Area is equipped with a leachate collection and removal system as opposed to the unlined Closed Active Area where generated leachate could pollute the groundwater. Because the unlined Closed Active Area was prematurely closed before final grades could be met, the surface drainage was not adequate to prevent ponding and stormwater infiltration into the buried waste, which eventually could generate leachate and contribute to groundwater contamination.

Searching for all possible opportunities to address the challenges of maintaining public safety from groundwater contamination while finding the resources to lessen the fiscal impact on City ratepayers, the NHIS program was developed.

NHIS materials are used so that the landfill can achieve final grade elevations and prevent surface waters from infiltrating the buried waste and creating leachate. As large portions of NHIS reach final grade, the landfill cap is constructed. This process is called a Rolling Closure. The combined use of the NHIS program, the monitoring program and the rolling closure process have allowed the Santa Maria Regional Landfill to be successful in achieving regulatory compliance while protecting the environment.

By working closely with regulatory agencies and environmental consultants, the City of Santa Maria is able to run the state-of-the-art NHIS project with operational efficiency and safety while generating significant revenue that helps to offset the need for landfill rate increases.

plastic liner

The NHIS is separated from the buried trash with a layer of clean soil and an industry-standard flexible plastic liner. This plastic liner adds an impermeable barrier between the NHIS and groundwater. The closure cap adds a second permanent barrier over the NHIS materials ensuring that surface water does not leak into the NHIS materials. The bottom plastic liner and top clay cap, encases the NHIS at the landfill.