A long term and growing concern in Colorado is river salinity levels and their impact on agricultural operations. The Colorado Ag Water Alliance recently invited Grady O'Brien, a hydro-geologist with more than 30 years of experience designing and implementing complex development projects, to discuss the early phases of a salinity research project he is conducting along the South Platte River Basin.

The entire webinar was recorded and will be posted at CAWA's website. O'Brien's preliminary report can also be downloaded and read on his firm's website at Neirbo.com.

"Salinity issues come up with a lot of producers I've talked to, and it is something we need to understand and try to get out ahead of," he said to start his presentation.

With grant funding from several collaborating organizations, O'Brien has begun studying all publicly available salinity readings along the South Platte from the Front Range to the Nebraska state line near Julesburg.

His conclusion is that limits on freshwater supplies have caused the salinity at all flows along the South Platte to increase over time, an impact that appears most pronounced in the upper basin. He believes Colorado needs to be looking at more aggressive mitigation strategies to protect such a vital and indispensable waterway.

As the "water gap" in the basin has grown — meaning the amount of water needed for growth and development has severely strained the limited amount of water available — re-use strategies to recycle existing water have become commonplace.

"The concern is, with all the water re-use, we could be increasing salinity in the water," OBrien said. "When you are pulling water out for reuse, the salts tend to build up. So is it a problem or not? That's the question."

He set the stage by reminding producers that some crops are more sensitive to salinity than others. Onions and dry beans, a popular crop in northeast Colorado, are highly sensitive, while wheat and corn are moderately tolerant, and sugar beets are one of the most salt tolerant options available.

Once salinity levels get above 1,000 milligrams of dissolved solids per liter, however, most crops will be affected, and yields will start to decline, he said.

Having more salt collecting in the soil also makes it harder for plants to extract water. In that case, dumping more water on a wilting crop "doesn't necessarily help, because the plants just can't draw water out of the soil. It leaves the crops stressed," he said.

These effects are compounded during dry years.

"If we have extended drought periods, it's kind of a perfect storm type of deal," he said.

In that scenario, overall stream-flows decline as demand increases, which prevents salts from getting flushed through the system.

At some point, excessive salt can leave soil permanently damaged.

At the beginning of the South Platte, where it exits the mountains, salinity concentrations are low, around 200 milligrams per liter. They climb to around 700 as it passes through Weld County, where it is still being influenced by mountain streams and tributaries that bring in freshwater from annual snow melt. But by the time it reaches the state border to the east, salinity levels, on average, are around 1,200 milligrams per liter. Across the system, levels of salinity are high in the winter and typically lower in late spring and summer when freshwater from the mountains dilutes it. Naturally, the lowest stream-flows have the highest concentrations.

Wastewater effluent from municipal and industrial water treatment facilities is suspected of being a major salinity contributor.

"The importance of wastewater effluent to the river's flow and water quality is magnified, because Chatfield Reservoir and other diversions remove the natural, low-salinity flow and replace it with higher-salinity effluent," O'Brien writes in his report.

Another key point from his presentation was that salinity levels vary over the course of the year, resulting in what he called "extreme salinity" events that routinely occur across the basin, but which aren't immediately apparent just by looking at the averages.

Spikes exceeding 1,800 ppm have been documented.

Farmers have already been forced to begin making changes. While they have the option of switching to more salt tolerant crops, that still has a negative impact on the agricultural economy as a whole, OBrien pointed out.

"You don't want to get into a mono-crop situation. Diversity is a good thing," he said. "If everybody switches to growing sugar beets, I don't think the farmers would be very happy about that."

Another common solution farmers have relied on for years was simply using more water to flush out or dilute the salt. "Historically, the solution has to been to apply more water," O'Brien noted wryly.

Greg Peterson, CAWA's executive director, talked about some of the ways farmers in the Imperial Valley of California deal with the issue.

"They apply a leaching agent after they are done for the season, so they are leaching the soil with an application of water," he said.

They also avoid crop input products with higher levels of salt, switch to furrow or drip irrigation methods where possible, and carefully manage irrigation timing and scheduling to avoid flooding any fields, he said.

Individual water treatment facilities should be more closely scrutinized to identify where the primary impacts are coming from, he said.

Water storage is another consideration. "Reservoirs and water recharge facilities, with a lot of them, the water is being put away during low-flow periods when salinity is the highest," he pointed out.

He also said road de-icing agents, which are heavily used in the upper basin of the South Platte, should be more closely examined.

Livestock feeding operations and oil and gas operations, which rely on produced water, are also possible contributors, although their influence appears to be minimal, he said.

He also called for more study of the regions natural geology, such as shale outcroppings, in hopes of better understanding of how they interact with the water management system in ways that contribute to salinity. Where possible, water should be stored and transferred without coming into contact with natural sources of salt, he said.

He and Peterson both noted that switching over from flood to sprinkler irrigation on many farms has helped to provide water quality benefits partly for that reason.

In his written report, O'Brien also emphasizes that evolving water law and water-management practices on the South Platte over the last decade have resulted in significant hydrological changes that still aren't fully understood.

As he writes in his executive summary, "Implementation of water law and water decrees has fundamentally changed the hydrologic system. Stream-flow patterns have changed, groundwater levels have risen, groundwater flow paths have been altered, and the magnitude and distribution of salt loads has changed."

O'Brien plans to continue his research if more grant funding becomes available. He has several goals for the next phase of the project, including more analysis of historical salinity levels and their correlation with crop yields over time; and a closer look at the implications of groundwater in-flow within the system and also back-flow from canals and reservoirs where surface water is stored.