Butte County Clerk of the Board

2008 BOARD OF SUPERVISORS MEETING CALENDAR

chapter three

scope and results of the Technical Program

This chapter summarizes the scope and results of a comprehensive technical program of groundwater nitrate characterization, hydrogeology, and soil chemistry conducted over a four year period. The overall objective of the program was to evaluate the extent to which individual septic systems were contributing to elevated groundwater nitrate. The results of the technical program could then function as the basis for further actions as needed to protect the beneficial uses of groundwater in the Chico Urban Area.

Butte County's technical program commenced in 1992 with an initial nitrate characterization by Metcalf & Eddy (Metcalf and Eddy, 1992), consisting of the sampling and analysis of 119 existing wells. This effort provided updated data and program recommendations for more extensive work to follow.

In September of 1993 a work plan was finalized by the Sacramento office of Dames & Moore, Inc., the hydrogeochemical consultant selected to conduct the bulk of the technical program. A second study phase was implemented with the close consultation of the Technical Team (see page 3-6) consisting of the Regional Board staff, the County, independent hydrogeologists and soil chemists and the public. A central element of the work plan was the installation of 19 monitoring wells in the shallow aquifer to provide more accurate characterization of the extent and severity of elevated groundwater nitrate. Upon completion of the work, Dames & Moore issued the Groundwater Nitrate Study Final Report in early 1994 (Dames and Moore, 1994).

Again, following extensive consultations among members of the Nitrate Study Team, Dames & Moore proceeded with an additional scope of work including; a nitrate source evaluation, further shallow aquifer characterization, and vadose zone characterization. The results of this study phase were set forth in the Technical Memorandum, Hydrologic and Soils Investigations, in April of 1996 (Dames and Moore, 1996).

3.2 Previous Studies

Several investigations were undertaken prior to the Technical Program, which was begun in 1992. Brief descriptions of each of the previous efforts includes:  Department of Water Resources Study, 1983. (California Department of Water Resources, 1984) Groundwater samples collected from 69 private water supply wells in 1979 first indicated elevated nitrate levels in 21 wells in the Chico Urban Area. In 1983, the Department of Water Resources conducted two rounds of groundwater sampling. The Department of Water Resources study concluded that nitrate levels exceeding the maximum contaminant level existed in four areas within the Chico Urban Area as shown in Figure 3-1. This study was the basis of subsequent actions by the Regional Board.

Aqua Resources Study, 1985. (Aqua Resources, 1984) A nitrogen isotope composition study of 22 groundwater samples was conducted by Aqua Resources to attempt to identify the sources of elevated nitrate levels in Chico Urban Area groundwater. The Aqua Resources Incorporated study concluded that much of the nitrate contamination was due to septic systems, including 70 to 90 percent of the nitrate contamination in the northern portion of the Chico Urban Area. However, the study was very controversial and largely discounted by later technical efforts, including those reported in the Dames & Moore Technical Memorandum.

Behnke analysis, 1989. (Behnke, Jerold, 1989) In conjunction with Aqua Resources Incorporated, Dr. Jerold Behnke conducted a nitrogen isotope study, concluding that agricultural operations were primarily responsible for the nitrate concentrations found by Department of Water Resources in 1984. Dr. Behnke suggested that d¹⁵N values (delta-15-N, a method of determining the source of nitrate, see glossary) were consistently too low to be from septic tanks as a primary source.

State Water Resources Control Board Review Committee evaluation, 1989. (Technical Advisory Committee, SWRCB, 1989)  The State Water Resources Control Board convened a Review Committee to evaluate existing data and determine if it was sufficient to evaluate nitrate sources, what impact the proposed prohibition order would have on existing and future groundwater quality, and what further information was needed. The committee's report confirmed that septic systems were substantially contributing to groundwater nitrate, as were other sources to a lesser degree.

Figure 3-1

Nitrate Plumes as identified by Department of Water Resources, 1984

3.3 recent technical program studies

In 1991, a review by Heritage Partners of the previous studies described above indicated that no comprehensive, coordinated and quality assured technical analysis existed with the scope and depth necessary to resolve the substantial controversy regarding the extent and source(s) of groundwater nitrate. With the ability to generate funds via CSA 114 assessments, Heritage Partners and county staff recommended that the County pursue a technical program which would form the basis of further actions.

In order to proceed in a prudent and cost-effective manner, an initial sampling of existing private wells in the shallow and intermediate aquifer was conducted by the Chico office of Metcalf & Eddy. In October of 1992, Metcalf & Eddy issued their Nitrate Characterization Summary Report (Metcalf and Eddy, 1992). The effort and data generated helped to shape the scope and direction of further, more definitive and costly studies.

Hantzsche Equation

The Prohibition Order was based upon the data available to the Regional Board at the time, which pointed to the effluent discharge from septic systems as a major source of elevated levels of groundwater nitrate. Cited in particular was the density of residential dwelling units and the unsuitable hydrogeologic conditions prevalent in the Chico Urban Area. The Regional Board specifically found that the protection of groundwater and beneficial uses could not be achieved with the continued discharge from onsite systems, even under management by an authorized public agency as provided in Section 13282 of the Water Code.

In concluding that the continued discharge from existing onsite systems was causing exceedance of the State standard, the Regional Board relied upon the application of an equation intended to predict the level of nitrate in groundwater. This "Hantzsche/Finnemore Equation" often shortened to "Hantzsche Equation", named for authors Norman Hantzsche and E. John Finnemore, estimates nitrate loading to groundwater based upon measured factors including rainfall/recharge, denitrification, soil chemistry and other factors. This equation is delineated in detail in Hantzsche and Finnemore, 1992 (refer also to Appendix G). This article was published in "Ground Water" a respected peer reviewed Journal. Dr. John Finnemore is a professor of Civil Engineering at Santa Clara University. Norm Hantzsche is a Professional Engineer in California, and principal of Questa Engineering in Point Richmond, California, and designs on-site wastewater treatment systems, both individual and community, as part of his consulting business. Application of the equation to a specific area, where these parameters may be reasonably measured and/or estimated, provides a "critical development density" expressed in acres per equivalent dwelling unit.

The goal of the County's technical program was to refine the parameters of the Hantzsche Equation, as applied to the Chico Urban Area. In so doing, it was hoped that a scientific definition of the sustainable density (i.e. the maximum density that will not cause exceedance of the maximum contaminant level) could be documented, and form the basis for a plan for compliance with water quality objectives.

If the amount of nitrate laden wastewater being discharged to the subsurface can be determined and the amount of rainfall and other recharge that mixes with this wastewater can be estimated, the resultant concentration of nitrate in this mixed water can be estimated. This method is very simple in concept, but somewhat more difficult to apply in practice.

First, the amount of nitrate discharged to the subsurface must be calculated. Fairly definitive knowledge is available regarding the amount of nitrogen that is discharged from septic systems. However, the literature estimates that have been used for this estimate vary by a factor of 2 to 3. In addition, the amount of denitrification that occurs below grade is subject to debate. In the case of the Chico Urban Area, site specific studies have been performed to pin these numbers down further. These studies show that a denitrification rate of 30% is a good estimate (Technical Memorandum, Hydrologic and soils Investigations for the Chico Urban Area, Dames and Moore, page 25, 1996), and that the average per capita production of nitrate is approximately 15 grams per day per capita (Kaplan, O. Benjamin,, Pages 148-149, 1988, State Water Resources Control Board, Identification and Evaluation of Methods for Determining Sources of Nitrate Contamination in Ground Water, page 18, 1994).

Next the amount of dilution water that is available must be estimated. For this study the methodology used was to take a look at the rise in the water level in 19 wells over several seasons. The value that was arrived at using this method was approximately 18 inches per year (Dames and Moore, 1996, pages 14-17) or approximately 2.5 times the amount used in the original calculation upon which the prohibition order relied.

The Hantzsche equation is as follows:

I x Nw x (1-d) + R x NbNr = ------------------------------  (I + R)

Where: Nr = Concentration of nitrate in the shallow groundwater (mg/L)

                I = The amount of wastewater entering the subsurface, averaged

                        over the gross developed property acreage (inches/year)

              Nw = The concentration of nitrogen in the wastewater (mg/l)

                d = The percentage nitrogen loss to the atmosphere due to denitrification (%)

                R = The average recharge per year (inches/year)

              Nb = The background nitrate content of the recharge (mg/L)

NOTE

Concentrations of nitrate in groundwater can be expressed as the weight of nitrate compound (NO₃ molecules, which consist of one atom of nitrogen and 3 atoms of oxygen, and atomic weight 62 units) or as the amount of nitrogen atom (the one atom of nitrogen, N, in a nitrate molecule contributes 14 units to weight of the molecule). Thus, for a nitrate molecule, the ratio of weight of the molecule (NO₃), to the atom (N) is 62/14, or approximately 4.5. Therefore, a concentration of 1 mg/l of nitrate expressed as the weight of just the Nitrogen (N) in the nitrate compound is equivalent to 4.5 mg/l of nitrate expressed as the total weight of nitrate (NO₃^2-).

A useful analogy may be to consider a 20 pound bag of 16-16-16 fertilizer (the first "16" in this designation refers to the percentage of nitrogen of the total weight of the bag). Such a bag contains 20 pounds of a combination of materials, of which approximately 3 pounds is nitrogen (16% times 20 pounds equals approximately three pounds). Therefore, regarding the amount of nitrogen in this bag it is equivalent to indicate that you have 20 pounds of 16-16-16 fertilizer or 3 pounds of nitrogen. Similarly, 4.5 pounds of nitrate molecules contains 1 pound of nitrogen.

For consistency, all references in this document are to nitrate concentrations expressed as the amount of nitrate compound (NO₃). To convert these concentrations as concentrations of atomic nitrogen, they can simply be divided by 4.5.

Technical Team

A team of scientists and engineers with expertise in this field was assembled in order to consult and provide guidance for the Technical Program. The County was fortunate to receive the professional services and consultations of the following key members of the "technical team".

Dennis Rolston, PhD - Dr. Rolston is a professor in soil science at the University of California at Davis, with extensive experience and publications in groundwater nitrate contamination. He sat on the State Water Resources Control Board Review Committee referenced above, and is recognized as an expert in the field.

Graham Fogg, PhD. - Dr. Fogg is a professor of hydrogeology at U. C. Davis, and is well-published on a wide range of hydrogeological subjects. Together with Dr. Rolston, he provided an ongoing "peer review", or double-check, on the efforts of the County's hydro-geochemical consultant, Dames & Moore.

Jeffrey Bold, PhD.-  Dr. Bold is a senior soil chemist at Dames & Moore, and has consulted extensively on groundwater contamination issues including the Koppers plant in Oroville and nitrate contamination in Davis. Dr. Bold was supported by hydrologists and other technicians at Dames & Moore.

Ronald Dykstra, P.E.-  Mr. Dykstra is a Senior Water Quality Control Engineer for theCentral Valley Regional Water Quality Control Board, with very extensive personal involvement in the Chico nitrate problem on behalf of the Regional Board.

Stewart Oakley, PhD.- Dr. Oakley is a sanitary engineer, wastewater consultant, and professor at California State University, Chico. He has presented many papers regarding wastewater issues and conducted the recirculating trickling filters study described later in this chapter. Dr. Oakley founded and heads the Environmental Laboratory within the College of Engineering and Computer Science at CSUC.

Scope of Technical Studies

The following program of work was conducted by Dames & Moore at the direction of the County and accompanied by the periodic review and discussion of the technical team.

Nitrate Source Evaluation

Shallow Aquifer Characterization

Vadose Zone Characterization

        ammonium, and total nitrogen

Dames & Moore conducted the resampling of over 100 existing wells and the installation and testing of 19 new sampling wells. The results were published in their Final Report, Groundwater Nitrate Study, in January of 1994. Based upon this refined and updated nitrate characterization data, the County pursued a study of potential sources of nitrate and the effects of soil chemistry on septic tank effluent. The results, set forth in the Technical Memorandum (Dames and Moore, 1996), confirm that septic tanks are a primary source of groundwater nitrate. However, the studies further indicate that soils in the Chico Urban Area have a higher capacity to assimilate nitrogen than was recognized by the Regional Board. Three rounds of sampling were conducted in December 1993, July 1995, and October 1995. The data generated by these multiple samplings and the evaluation of further data needs prior to each collection, greatly increased the technical team's understanding of the extent and nature of the nitrate contamination.

Groundwater elevation was measured at each sampling. The general direction of groundwater flow, shown in Figure 3-2, (Modified from Dames and Moore, 1996) is a basic factor in reaching eventual conclusions as to nitrate sources. In general, water level elevations are highest in the north and east portions of the Chico Urban Area, and lowest in the west and south. Depth to the water table from the ground surface ranged from approximately 11 to 45 feet below ground surface depending upon location and season.

Nitrate values at each of these samplings showed a range from background levels of less than 10 mg/L to various readings above 100 mg/L. Figure 3-3 shows nitrate concentrations in the shallow aquifer. One undeniable characteristic is that nitrate concentrations are highest directly down-gradient (westerly) from the unsewered portions of the Chico Urban Area. Further, concentration is highest down gradient from the greatest number of onsite systems, along the "Lassen Avenue Corridor".

Nitrate concentrations in the intermediate aquifer are shown in Figure 3-4. As with the shallow aquifer, nitrate concentrations are greatest down gradient from the areas of septic tank usage. It is worth noting that these nitrate values represent the conditions at a specific point in time.

The recent efforts in nitrate characterization involved a greater emphasis on wells of known construction, multiple samplings and very careful quality assurance control. Nevertheless, the results of the recent work are generally consistent with past data--continued elevated nitrate levels (above the maximum contaminant level) westerly of the unsewered portion of the Chico Urban Area.

d15N Isotopic Analysis

One of the more sophisticated methods used to determine the source of nitrates in groundwater is nitrogen isotope comparison. This method is described below.

Nitrogen exists in two primary forms in nature, one form has 7 protons and 7 neutrons, the other 7 protons and 8 neutrons. Therefore the former form has an atomic weight of 14 (7 + 7), and the latter has an atomic weight of 15. In the atmosphere, there is a constant ratio of the two forms. However, when nitrogen from the atmosphere is taken up by plants, or fertilizer is manufactured, or any other assimilation of nitrogen takes place using atmospheric nitrogen, one form tends to be used in the process differentially. In other words, more of one form will be used than is proportional to its natural abundance in the atmosphere, and the difference is measurable.

By studying the different processes that use nitrogen, it is possible to measure these differences, or "signatures", of the different origins of nitrogen compounds. In other words, one process, such as the manufacture of fertilizer, will result in a ratio of the two forms of nitrogen that identifies the source as fertilizer, whereas another process, such as dairy production, will produce a different signature which identifies it as livestock waste.

During the course of the studies that were performed in the Chico Urban Area to determine nitrate sources, both literature values and local site specific values were studied. The information obtained was consistent with the primary source of nitrate in the Chico Urban Area being from septic tanks (Dames and Moore, 1996, page 13).

Figure 3-3

NITRATE IN THE SHALLOW AQUIFER

Figure 3-4

Nitrate in the Intermediate Aquifer

Evaluation of Potential Nitrate Sources

The following were identified as potential sources of nitrate and were evaluated for relative contribution to groundwater:

Soil Nitrogen. Soil nitrogen was not found to be a major source of nitrate. For further discussion see Section 3.4.

Background nitrogen in groundwater. Although somewhat inconsistent, background nitrogen in groundwater upgradient from the Chico Urban Area is shown to average 2.15 mg/l, and is an insignificant source.

Background contribution from surface water. The results of five of six downstream surface water samples suggest that urban runoff to surface water is not a source of nitrogen in the Chico Urban Area. (Dames and Moore, 1996, page 23).

Historic animal agricultural sources. A concerted effort to research potential agricultural sources was conducted, consisting of a review of aerial photographs, interviews with individuals familiar with agricultural operations dating back to the 1920's, and other relevant methods. Although historic animal agricultural land uses with significant nitrogen loading were identified, none were identified that could explain the current nitrate levels. Figure 3-5 represents a summary of the findings of this research.

Regarding area animal operations which were generally terminated by the 1960's, Dr. Bold of Dames & Moore makes the following conclusions regarding historic agricultural land uses (Dames and Moore, 1996, page 8):

Figure 3-5

Historical Land Use Report

Orchard and crop fertilization. The d¹⁵N values of fertilizer were found to range from 2.4 to 4.33, consistent with other studies of d¹⁵N. Three separate rounds of sampling and analysis of water from the shallow aquifer showed values from 7 to 10, much higher than fertilizer (Dames and Moore, 1996, page 10).

Industrial nitrogen contribution. No current or past industrial land uses which might generate significant amounts of nitrogen were identified (Dames and Moore, 1996, page 1).

Discharge from septic systems. Application of the refined factors of the Hantzsche Equation shows that existing residential densities are discharging levels of nitrate that exceed the soil's ability to assimilate. Areas of elevated nitrate are located generally down-gradient from concentrations of high-density residential development on septic systems. d¹⁵N values were found to be consistent with septic tank effluent sources, and not consistent with other sources (Dames and Moore, 1996, pages 25 & 26).

3.4 Conclusions as to Nitrate Loading from Onsite Systems

The goal of investigations performed by Dames and Moore, qualified professionals experienced in hydro- geochemical evaluations, was to evaluate all available information on the sources of groundwater nitrate, including information from site investigations. The site work performed by Dames and Moore included drilling, developing, and sampling 19 wells, sampling existing wells, installing and sampling lysimeters at existing leach fields, and performing soil borings accompanied by soil sampling near several leach fields.

Dames and Moore's conclusions regarding the major potential sources of nitrate in shallow groundwater are reiterated, as follows (all page references are to the Technical Memorandum of Dames and Moore, 1996, unless otherwise noted):

Single and multiple-family dwellings using septic systems for managing wastes are an important source of nitrogen which is impacting groundwater (page 25).

3.5 Refinement of the Hantzsche Equation

As discussed previously, the Hantzsche Equation is useful in predicting groundwater nitrate from septic systems and was applied by the Regional Board in imposing the minimum of one dwelling unit per acre. Based upon the information available at the time, and reasonable estimates made by Regional Board staff, the values for recharge, denitrification and other factors were set, and a substantial safety factor applied.

One of the objectives of Dames and Moore's Technical Memorandum (Dames and Moore, 1996) was to develop better data to refine some of the parameters of the Hantzsche Equation, especially those of denitrification and recharge. It is difficult to obtain site specific information on the remaining parameters of the equation (excepting dwelling occupancy) without extensive and very expensive sampling. Therefore, although some site specific data was available and was consistent with the parameters used in the equation (as noted in the references below), heavier reliance was placed upon literature values for the remaining equation parameters.

Dames and Moore's studies indicated reasonable values for recharge and denitrification of 18 inches per year and 30% respectively (pages 17 and 25). Census data for the Chico Urban Area indicates the average occupancy per dwelling is 2.43 persons. Therefore, the Hantzsche equation (presented on page 3-5 and also in Appendix G) was used with the following parameters:

43,560 cu ft/acre ft x 7.48 gal/cu ft x (1 ft/12 inches)

15 g/day x 1000 mg/kg x 0.8 (assume 20% reduction in tank) = 40 mg/l as N (nitrogen)

Nw = -------------------------------------

80 gal/day X 3.78 l/gal

Therefore;

[(10.45 in/year) x (40 mg/l as N) x (4.5g NO₃/1g N) x (1-0.3)] + [18 in/year x 0.5 mg/l as NO₃]

Nr = ----------------------------------------------------------------------------------------------------------------------------

(10.45 in/year + 18 in/year)

Nr= 46.6 mg/l as NO₃ (nitrate)

The solution of the equation therefore shows that dwelling densities of four units per acre will slightly exceed the nitrate maximum contaminant level. This is corroborated by Dames and Moore's Technical Memorandum, which states "The impact of single-family dwelling units (DU) at low to moderate densities (less than 3 to 4 dwelling units per acre) may be attenuated by the relatively high recharge conditions within the Chico Urban Area, and the denitrification which occurs 5 to 15 feet below ground surface underneath septic leach lines (page 26)".