This feature class depicts the boundaries of Operable Unit 1 (OU1) within the Carson River Mercury Site (CRMS), including the four Area of Investigation boundaries.

NDEP lead an initiative to refine the boundaries shown on maps identifying the Carson River Mercury Superfund Site (CRMS) from the initial site identification and description as the Carson River hydrographic basin beginning in Carson City, NV to its terminal points in Churchill County, NV. This description of the CRMS was used in many of the early site investigation studies and reports and continued to be used in all public education and long-term site management controls until approximately 2012. Using site contaminant fate and transport determinations from the CRMS OU1 Remedial Investigation (RI) and Conceptual Site Model (CSM), it seemed inappropriate to include the much larger area of the hydrographic basin as being potentially impacted by site contaminants of concern (CoC). NDEP created protocols to estimate areas that are likely to have been impacted by CoCs and created maps using these protocols to redraw the CRMS boundary limits.

In the development of these protocols, NDEP used: Historic records and documentation of probable source areas; The 2012 archaeological mill site research and field study conducted by Broadbent and Associates to locate the historic source areas; Soil and sediment transport mechanisms identified in the CSM and general soil transport and sedimentology principals to predict areas where CoCs have likely been located at and downstream of the historic sources.

The overall area of transport was estimated to be reasonably large to include possible anthropogenic activity as well as historic and future natural events such as flooding and channel migration. Additionally, NDEP added a "buffer” to extend the potential areas beyond the conservatively defined primary areas of potential contamination to further address unknown and future effects. These buffers have been identified separately from the primary areas of concern and labeled as such on maps using these protocols. These revisions were formalized in the 2013 Explanation of Significant Differences to the OU-1 Record of Decision (RoD) to adopt the new site definition and boundaries identified by NDEP as new estimates of the CRMS extents.

NDEP identified four geographic areas of prime importance to the CRMS. Originally labeled by NDEP as "Risk Areas”, the intent of this terminology was to indicate these were the areas understood to have the most likely "risk” of contamination. Under consultation with EPA R9 risk assessment staff, the term has been changed to "Area of Investigation” to avoid confusion that any actual quantitative level of human or ecological risk has been determined for these areas. They are only estimates of potential contamination and new or additional information that contradicts these boundaries as being insufficient will be used to adopt new boundaries as appropriate. The four Areas of Investigation have become the foundation for the residential soil sampling program mandated by the OU-1 RoD. Summarized briefly;

Investigation Area 1 (IA 1)- This includes all areas in the Carson River drainage basin from about the area of the historic settlement of Empire in Carson City, NV downstream to the existing or historic terminal points of the river at Carson Lake, Carson Sink, Indian Lakes and the Stillwater National Wildlife Refuge that lie outside the buffer zones of the other three Investigation Areas. It is least probable that CRMS CoCs will be located in these areas. It is unlikely that any sampling will be requested on areas developed within IA 1, but it might be requested in special circumstances, especially near and around the source areas of the contamination where historic activities could potentially have caused contamination beyond the typical boundaries as identified by the CSM.

Investigation Area 2 (IA 2)- This area is defined as a buffer that lies 100 feet along a normal horizontal to the Investigation Area 3 boundary. For Comstock-era mill sites and isolated tailings piles, this translates to the area between 350 feet and 450 feet from the center point of the historic feature. For the 100-year FEMA floodplain and areas of irrigation, this is the area beginning at the limit of the flood plain boundary or irrigated land along a normal to 100 feet.

Investigation Area 3 (IA 3)- This area is defined as a buffer that lies 100 feet along a normal horizontal to the Investigation Area 4 boundary for Comstock-era mills or isolated tailings piles. For Comstock-era mill sites and isolated tailings piles, this translates to the area between 250 feet and 350 feet from the center point of the historic feature. It is also defined as the limits of the FEMA 100-year floodplain or past or current flood irrigation practices. A tributary of the Carson River must have a Comstock-era mill site or tailings pile located along it to be mapped in IA3 and only the portion of the tributary downstream of the historic feature is included, not including the IA3 area and buffer drawn around the historic feature itself. If a tributary does not have FEMA 100-year flood plain defined, then IA3 has been defined as the area 100 feet along a normal to Investigation Area 4 boundary of that tributary.

Investigation Area 4 (IA 4)- This area represents the highest likelihood of mercury contamination. Multiple steps were used to define the extent of this area, described below:

The area within a 250 feet radius from the center point of a Comstock-era mill Comstock-era tailings pile polygon with a 250 ft buffer The centerline of the current channel of the Carson River enclosed in a polygon 100-feet wide (50 feet either side). Tributaries to the Carson River, where Comstock-era mills and tailings piles were located are enclosed in a polygon 50-feet wide (25 feet either side) from the approximate center of the tributary channel. Irrigation canals are enclosed in a polygon 20 feet wide (10 feet either side) from the approximate center of the ditch.