In our previous article, we looked at where your wastewater goes by talking about discharge options for disposing wastewater to land. In this article, we’ll answer the question “Why should I care about my wastewater?” by discussing the effects that wastewater application can have on the receiving environment.
And we know you may be saying something like “I have an environmental license, I have to care about my wastewater or I’m going to get fined!” and while this is true, there are generally reasons for the limits set in the license. Knowing why those figures are set, what affects the land itself and how, and what can happen if you do not comply with the requirements can help you in maintaining your treatment plant and irrigation area. We’ll answer the less obvious questions here; how can the quantity and quality of my wastewater affect my irrigation area?”
If too much effluent (output from the wastewater treatment process that is released into the environment) is applied too quickly, it can cause the irrigation area to “overflow”, exceeding it’s capacity and creating surface runoff. If this happens, effluent may be released from the irrigation area to the surrounding environment via surface water flow. Additionally, exceeding the water holding capacity of the irrigation area may cause effluent to discharge through the soil to groundwater (through excess soil infiltration), which can lead to potential groundwater contamination.
This is not good and in both cases, you would be dealing with potentially serious breaches of environmental compliance.
Let’s take a look at the diagram below:
For the irrigation area to work effectively, the general requirement is for the rate of total evaporation, transpiration (uptake through plants) and soil storage to exceed the rate of irrigation. This ensures there will be not be either pooling of wastewater on the irrigation area, or effluent discharge to groundwater.
The factors normally affecting the ability of land to accept wastewater are:
Site exposure to sunlight and general climate;
Plant and grass coverage of the irrigation area;
Properties of the soil; and
Site slope and landform.
Site exposure to sunlight and your climate can’t generally be changed, and will need to be accounted for in the design of wet weather holding faculties and the overall size of the area. Plant and grass coverage may be able to be manipulated by specifically planting the irrigation area. This can be expensive but the difference between plant types can be large. The soil properties have a large impact upon the irrigation area, and can drastically effect the tendency of an irrigation area to pond, or to discharge to groundwater.
Soils with higher drainage rates (hydraulic conductivity) can accept more wastewater without ponding, but may cause excessive infiltration to groundwater at higher application rates, while soils with lower hydraulic conductivity can cause effluent ponding at higher application rates. Soils are normally layered, with topsoil overlying other soil types (such as clays, rock, etc) down to bedrock. Changes in the conductivity of each soil layer can effect effluent movement through the soil, which in extreme cases can result in effluent moving along a soil horizon (generally at a rock or other impermeable interface). This results in the effluent breaking out at the base of slopes, hills or weeping from the side of embankments.
Finally, the shape of the land will affect the potential or tendency for applied effluent to cause runoff; greater slopes can cause excessive runoff even at comparatively low application rates, and specific landforms (e.g. concave hill slopes) can cause concentration or ponding of irrigated effluent, again at comparatively low application rates.
A site and soil evaluation (SSE) is normally carried out by a professional consultant (usually an engineer or soil scientist) and aims at describing the site, it’s vegetation and the soil type and the shape and formation of the subject area.
This allows for the recommendation of a suitable irrigation rate based upon both experience and standards. A SSE should be done before establishing an on-site effluent disposal area and, ideally, should be conducted before designing the treatment plant itself. The ability of the land to accept wastewater should also factor into the design of any emergency wet weather facilities and the control of the irrigation system.
Now that we’ve discussed effluent quantity, let’s consider the quality of the effluent.
Typical wastewater is comprised of a variety of waste constituents. Some of these constituents, primarily Nitrogen and Phosphorus, can be beneficial to plant growth in the right concentrations, conversely they’re also harmful to plant growth if you have too much. In addition to impacting plant growth, too great a concentration of wastewater constituents can lead to soil contamination, the contamination of runoff, or the contamination of groundwater. Ideally, the irrigation area is sized such that the vegetation that the effluent is applied to takes up the nutrients applied and uses them for growth.
The key nutrients (macronutrients) required for plant growth are:
Micronutrients (which aid plant growth, but are not necessary) include:
Plants and vegetation will grow poorly if there is an imbalance of macronutrients. This is not only harmful to the vegetation in the irrigation area, but also prevents effective transpiration, which is the process by which water moves through plant and evaporates from parts exposed to air, such as leaves, stems and flowers. Below you’ll see a diagram which shows the the effect of nutrient deficiencies.
To further complicate the situation, nutrients are only available to the plants and vegetation if the soil meets the ideal pH range for that nutrient. The next figure shows the ideal soil pH for nutrient availability.
To assess whether an irrigation area is healthy, monitoring is normally organised to take soil and plant samples on a regular basis (typically yearly). Ideally these are compared to a base-line sample taken before the establishment of the irrigation area. This allows us to check to see if irrigation activities are impacting the plants or soil, and is normally required under the environmental authority for the site.
When assessing an existing irrigation area, the condition of grass and vegetation covering the effluent area is assessed. Stressed or damaged plants could be showing leaf discolouration or leaf scorch, or poor growth in general. This provides a relatively rapid assessment of whether an area is over irrigated, under irrigated, or if there are nutrient deficiencies in the wastewater.
Baseline samples are collected prior to any irrigation, for comparison with soil results collected after short or long-term irrigation practices. Alternatively, a “control zone” may be established. Both options give a good indication of the natural condition of the local soil, although control zones should only be used if there is no variation in soil type across the site.
After soil and plant samples are collected from the active irrigation area, they are analysed at a laboratory to check the balance of cations (macronutrients) and anions (micronutrients) in the soils and plants. These samples are compared to either control zone or baseline values, and ideal cation and anion ratios. The S&B Consultancy Team uses a range of commonly-referenced metrics and measures to get a better idea of the soil quality and ability for plants to uptake nutrients.
Soil sampling results show the amount of plant macronutrients and micronutrients available within the soil profile, as well as other contaminants in the soil. For instance, high sodium concentrations can indicate that a soil is sodic, which means that the soil loses its structure (disperses) when wet. In an agricultural context this typically leads to poor soil permeability due to swelling, which restricts the movement of water in the soil. Sodic soil is typically indicated by soil which has cracked from swelling open and can lead to poor crop growth. You can see what sodic soil looks like in the photo below.
The laboratory results allow a suitably experienced professional to pinpoint the specific nutrients or causes of poor soil and plant quality.
So, now let’s answer our question “Why should you care about your wastewater (beyond the obvious)?” Simply, we care because the effluent going to your irrigation area can cause significant environmental damage. Nutrients in the sewage can degrade soil structure or prevent plant growth on the disposal area, which can eventually result in groundwater contamination from infiltration, or surface water contamination from runoff. Exceeding the water holding capacity of the irrigation area can also cause environmental contamination through runoff or infiltration.
So we’ve answered the question, but what can we do about it?
The best way to deal with poor soil or plant quality is to have a properly designed and run irrigation system in the first place. This design should not be limited to just the disposal area, but should include proper wet weather storage facilities, application controls and application techniques. We’ll talk more about this in an upcoming article.
Appropriate design before initiation of irrigation helps to alleviate many problems with soil contamination (such as sodicity), as well as problems with poor plant growth due to nutrient deficiency or poor soil pH. The use of modelling software such as MEDLI can aid in irrigation feasibility studies and designs (Simmonds & Bristow have significant experience with the MEDLI model, and you can read more about MEDLI here or visit our webpage on modelling here). Other modelling software, such as Hydrus, can be utilised to provide further, more specific testing of irrigation areas, allowing for more detailed modelling of the movement of water through the soil (eventually to groundwater) and active and passive nutrient uptake by plant life. Correct operation is also important.
If the soil or plant quality in an effluent disposal area is already poor, it is often best remedied by further treating the effluent prior to irrigation, or modifying the irrigation rate, allowing natural processes (such as rainfall) to repair the soil. This could mean improving or adding treatment processes, reducing the volume of effluent disposed, or increasing the effluent disposal area. A number of agricultural fixes may be possible (planting specific plants, applying soil treatments and fertilisers, etc.). However, as these can be quite expensive to apply we stress again the benefits of planning ahead for effective irrigation.
Simmonds & Bristow can provide expert advice in all matters related to effluent disposal and irrigation. From field sampling, to soil and plant analysis, to assisting with plant upgrades, we are able to help you with any irrigation issues.
Street Address 2/40 Reginald Street Rocklea, QLD 4106 Australia
Postal Address PO Box 849 Archerfield QLD 4108 Australia