Nitrate levels in Barton Springs have been increasing in recent years. And, according to a USGS report, a good deal of it is from wastewater sources. Who could have guessed? I mean, besides anyone who gave this matter a moment’s notice.
The graphic above shows the growth of those wastewater sources from 1990 to 2010. The top line of graphics shows the growth in number of OSSFs. That stands for on-site sewage facility, the rules-speak name for what are popularly known as “septic” systems. The bottom line shows TLAP systems. That stands for Texas land application permit. In this type of wastewater system, the effluent is spewed out over an area that is “irrigated” mainly just to make the water go “away” rather than for an actual irrigation benefit, such as an improved landscape or growing a marketable crop. In the most used type of “septic” system – consisting of an “aerobic treatment unit” (ATU) and a couple spray rotors – the wastewater is also spewed out over the ground, with little regard for the value of the water, or of the environmental impacts. Such as increasing the nitrate levels in Barton Springs.
As these graphics show, the density of the “septic” systems has increased very dramatically over the 2 decades they cover, and the number TLAP systems has also greatly increased. Given the nature of those systems, spewing the water over land surfaces that, in the case of OSSFs, are not “qualified” at all and, in the case of TLAPs, are addressed in a rather cursory manner, it should not have been the least bit surprising that nitrate levels would be rising in the waters that drain out of this watershed. Indeed, particularly when combined with increases in pollution it was known would occur simply because development was occurring there, it should have been readily anticipated that this would be so.
The level of nitrate in Barton Springs is approaching 2 mg/L. The often quoted limit for nitrate in drinking water is 10 mg/L. This is what is termed the “enforcement limit”, the level at which definitive action would be required to reduce nitrate in groundwater. But there is another limit in the rules, 2 mg/L, which is termed the “preventative action limit”. That is the level at which actions to stem the shedding of nitrate into the groundwater – preventative action – are to be considered. We are there! So it’s time to start taking preventative action, no?
The tragedy here is that this did not have to happen. Preventative action has been available all throughout those 2 decades. Wastewater could have been managed by means which would have greatly blunted, perhaps essentially eliminated, the shedding of nitrates from these wastewater sources. AND this could have been done at very low overall cost, perhaps at NO cost – or even at a savings – in terms of global life-cycle costs of this water management function. In any case, the tide can certainly be turned going forward by moving practice to those methods.
First, here is what is wrong with the currently prevailing methods. The ATU employs a technology, activated sludge, which is inherently unstable, and so typically suffers “excursions” in its treatment quality when used in the essentially unsupervised on-lot environment. In any case, the design of the treatment unit does not remove nitrogen from the wastewater. Spraying this effluent onto the ground surface also limits the amount of denitrification – the biologically-mediated conversion of nitrate to nitrogen gas – attained in the soil. These on-lot systems spew the effluent onto the ground without regard to whether it’s raining or how wet the ground is. The result is that a nitrate-rich effluent is dispersed in a manner that heightens the likelihood a good bit of it would be shed, rather than assimilated in the plant/soil ecology, and so would appear in the waters that drain from this watershed.
That shedding of nitrate can be greatly blunted, perhaps even essentially eliminated, by a shift in the type of OSSF used. First, a treatment unit employing recirculating gravel filter (RGF) technology can be designed to remove a majority of the nitrogen from the wastewater prior to dispersal. This is a very robust, inherently stable treatment process, so consistently produces this high-quality, denitrified effluent in the on-lot operating environment. The major proof-of-concept field study of this technology was a project I ran on Washington Island, Wisconsin, in which nitrogen reduction of over 60%, and in some cases approaching 90%, was consistently achieved by systems subject to all the vagaries of operating in the on-lot environment. So using the RGF instead of the ATU for treatment will eliminate over half the nitrogen loadings prior to dispersal, consistently and reliably.
Then this effluent can be dispersed in a subsurface drip irrigation field. With the level of nitrogen in the effluent reduced, it is much more evenly matched to the uptake rate by plants. This dispersal method will also enhance in-soil denitrification. Together, these assure consistently more complete assimilation of the nitrogen that is dispersed into the soil. And subsurface dispersal eliminates runoff of the effluent during rainy weather. The result is that very little nitrate will leach or flow “away” to appear in the waters that drain from the watershed.
Drip rather than spray dispersal also can greatly serve the water economy by displacing potable water with this effluent to defray irrigation demands. Because spray dispersal entails a potential for contact with this partly treated water (which is also questionably disinfected, the reasons for which we won’t get into here), the spray heads are set away from the house, generally off somewhere on the lot where they won’t be “obtrusive”. But the improved landscaping, the plants that might be irrigated in any case, are typically up around the house, so these spray systems are hardly ever arrayed to serve that landscaping. Because the drip lines are subsurface, there is very low contact hazard, so the water can be dispersed anywhere on the lot where the owner chooses to install irrigated landscaping, and the effluent would defray irrigation usage, pretty much gallon for gallon.
Also, irrigation efficiency of drip is much greater than for spray. In any case, the rules require the design dispersal rate for spray systems to be very low, so not much irrigation benefit could be derived even if it operated at higher efficiency. The rules allow the application rate for drip to be significantly higher, much more in line with irrigation rates through the peak irrigation season. So, in combination with the high irrigation efficiency of drip, a much higher irrigation benefit can be derived from drip dispersal.
Further, the rules do not require the area over which effluent is sprayed to be “qualified” in any meaningful way, in regard to soil depths and plant cover. In contrast, drip fields must have at least 6 inches of soil beneath the drip lines and 6 inches of cover over them. In Hill Country terrain, this often requires importing soil to attain these depths. Soil is often imported to create improved landscaping in any case, so with drip the OSSF dispersal field is typically placed in the best soils available on the lot. This also increases irrigation efficiency, and with there being more soil volume to “absorb” the water even when the soil is already wet from rainfall, provides for better assimilation of nutrients.
The bottom line is that with higher quality pretreatment, including significant nitrogen reduction, and drip dispersal, the shedding of nitrate would be greatly blunted, if not essentially eliminated, and a very high percentage of the annual effluent flow could contribute to defraying water used for irrigation. The first benefit would halt whatever portion of the nitrate increases in Barton Springs that have been due to OSSFs. The second benefit is a bonus, one that is very valuable to this water-challenged region. I’ve been designing this type of OSSF for over 20 years, and it has been approved by all the local jurisdictions. Therefore, these are benefits that can be readily realized, that could have been attained all along.
So why weren’t they? We won’t belabor the details here, but the installed cost of the RGF/drip irrigation system would be somewhat higher than an ATU/spray system. And that’s why the latter are so ubiquitous, because first cost typically rules the day. However, the life-cycle costs would be similar, at least if the cost of the water saved is taken into account. Whether a direct cost is incurred for that water would depend on if the home were served by a well or by a piped water system. Other savings derive from much lower power costs (also a benefit in regard to energy sustainability), from lower equipment replacement costs, and from not requiring chlorine for disinfection (another insult to the environment that is avoided by subsurface drip systems). So nitrate reduction could be realized at very low, or no, cost on a global, life-cycle basis. Again, the barrier is first cost.
These same technologies could be just as readily used in those TLAP systems. In those systems, the land application is, in theory, operated so that nitrogen loadings match plant uptake and in-soil denitrification rates. That could be much more readily, and cost efficiently, attained using the denitrifying RGF system for treatment and subsurface drip irrigation for dispersal. The shedding of nitrate could be further attenuated by placing the drip fields in areas that would be irrigated in any case – again, these would typically have better soils than the rangeland and cedar breaks typically constituting the dispersal field in TLAP systems.
Going forward, then, further increases in nitrate being shed into this watershed can be blunted by shifting to the appropriate technologies. Over time, the existing sources could also be phased out. As people come to value the water being thrown away in their sprayfields, the spray systems may be replaced with drip irrigation fields, arrayed to irrigate their highest value landscaping. This could be spurred on if there continue to be water curtailments due to drought, since the drip field would drought-proof the landscaping it serves. That’s because water curtailments in all the local drought contingency plans impact only exterior water use. The wastewater dispersed in the drip field would derive from interior water use, which is not curtailed, so that landscaping could continue to be irrigated unabated through the drought. In any case, as noted previously, the drip field would more completely assimilate nutrients in the effluent, so reducing the amount shed.
Then too, as the ATUs wear out, or the owners get tired of the frequent replacement costs (or the stench they often produce), they could be retrofitted to an RGF, obtaining the nitrogen reductions in the treatment system as well. Together with the drip field replacing the spray system, again this would greatly blunt, if not essentially eliminate, the nitrate being shed by the existing OSSFs.
This is a fairly impressive list of benefits, for both water quality and water quantity, from simply plugging in the appropriate technologies for the circumstances at hand. As noted, the barrier is the first cost of the appropriate technologies, along with the inertia of the field, and the sad fact that ATU/spray systems are accorded what amounts to a “most favored status” in the OSSF rules system. The latter two factors are matters of reforming the “culture” of the OSSF field, but the first cost issue is a ubiquitous problem in regard to all manner of efforts to enhance water sustainability. Society has not figured out how to send to those who incur the first costs the signal sent by the global life-cycle costs. The result is that choices are made which well serve the short-term interests of those who bear those first costs but poorly serve the long-term best interests of society.
A solution to that conundrum would be appropriate regulation to attain the ends which do serve the long-term best interests of society. Like requiring OSSFs in nitrogen-sensitive watersheds to meet nitrogen reduction standards. Here in Texas, society has not yet gotten around to considering its long-term best interests in this regard. So we’ve seen, and no doubt will continue to see, increases in the level of nitrate measured in Barton Springs.