This page provides a general interpretation of existing methods for designing spray irrigation facilities, and also considers the relative effectiveness and limitations of these facilities.

Many of the topics listed below are referenced back to or provided as interpretation of requirements found in Delaware’s Regulations Governing the Design, Installation and Operation of On-Site Wastewater Treatment and Disposal Systems (7 DE Admin. Code, 7101).

The information provided below applies to Class C designers, Class D.3 soil scientists, Class E.4 system contractors, and spray irrigation professionals for the siting, designing, installing, and operating requirements of spray irrigation facilities.

General Interpretations

Compliance Monitoring Report, Nitrogen Balance, and Operations and Maintenance Manual

• Requires Class C designer to sign and seal each upon submittal.

BOD, TSS, pH Exceedances

• Monthly exceedances will be handled on a case-by-case basis when occurring with algae blooms in ponds or other potential impacts; however, a non-compliance notification (NCN) must be submitted with an explanation to this affect.

Daily Permissible Average

• Means the average of daily samples taken over the course of any given month.

Lysimeters (pan and suction)

• The Groundwater Discharges Section is authorizing the use of either pan or suction lysimeters.

Plow-Down Non-Creditable

• There is a net-zero effect for cover crops if not harvested or removed from the site.

Crop Nutrient Removal vs Crop Nutrient Uptake

• Crop Nutrient Removal is defined as the total amount of nutrients removed from the field in the harvested portion of the crop (e.g., grain, silage, hay).
• Crop Nutrient Uptake is defined as the total amount of nutrients contained in the entire crop at maturity (e.g., in the grain, stover, and roots).

Crop nutrient removal is lower than crop nutrient uptake because a significant percentage of the nutrients taken up by the crop are returned to the soil in the form of crop residues.

Design Factors and Generally Accepted Values

Commercial Fertilizers

• For design purposes related to nitrogen balance, assume 75 percent of nitrogen is utilized the first month of application and the remaining 25 percent is utilized the next month.

2.5 inches/week spray vs. monthly average

• Regulations stipulate a maximum of 2.5 inches/week.

90-Day Evacuation

• The design of new and expansion facilities must demonstrate the ability to evacuate all accumulated and current flow within a 90-day period once the maximum storage accumulation has occurred and active spraying activities begin to ensure proper sizing of the facility. Operationally, this will be handled on a case-by-case basis for those facilities not evacuating their lagoons by late summer.

Nitrogen Fixation

• For soybean and alfalfa nitrogen fixation values, assume 60 percent of crop needs are derived from the soil and 40 from the atmosphere.

Precipitation – P5 vs. Average Values

• P5 values are to be used in water balance calculations while P-average values are to be used in the nitrogen balance percolation volume calculations.

Crop Growth/Uptake Curves/Standard Design for Crop Removal

• Perennial Grasses, Golf Courses/Sports Complexes, Row Crops/Soybeans, and Wooded/Forested Sites

See Crop Reference (EXCEL)

Spray Allowance (language in permit)

Below is a sample of what to expect in a permit, possibly located in Part I in a new section entitled “Design Criteria.”

During the period beginning on the effective date and lasting through the expiration date of this permit, the Permittee is authorized to discharge to the spray irrigation field(s) identified on page 1, in Part I.A, and depicted on page X of this permit the quantity and quality of effluent specified below and in accordance with the design documents listed in Part I.B of this permit:

1. The design loading rate proposed is a maximum of XXX,XXX GPD in accordance with the XXXX Design Engineer Report  (see Design Loading Rate chart below). If facility operations face emergency or extenuating circumstances that would require the weekly quantity of effluent discharged to exceed 2.5 inches per acre per 7-day period, and the system would be able to assimilate both the hydraulic loading without exceeding the soils percolation capacity and the additional nitrogen loading without exceeding the limitations set forth in item 11 below, please contact the Groundwater Discharge Section for written authorization in accordance with Section 6.3.2.3.13.8.1 of the Regulations.

Related Topics:  clean water, groundwater, permitting and regulation, spray irrigation, water