Standing in a sea of green as a fiddle whistled and a bass thrummed on the stage, my friend turned to me to ask if I wanted another beer. I was tempted. Celebrating St. Patrick’s Day the way I had when I was twenty-five, dancing drunkenly in the alley of an Irish pub downtown, I liked the idea of keeping the debauchery going.

However, I was even more stoked about the idea of going to the ocean-friendly garden event the next afternoon. I guess that’s what it means to be thirty-four, so I opted out and took myself home.

The next morning, I woke up, grabbed some coffee and walked to the Muni Station to catch the “N” to the Outer Sunset. 

The final ocean-friendly garden event

At 43 and Judah, I hopped off the “N” and walked to the Far Out West Community Garden. With an eye-catching mural along one side of the plot, this garden is hard to miss. 

At the far end, an array of chairs were set up beneath the shade of a large tree. Most of the attendees were already present, so I hurried over and found a seat. 

Shortly after, we started the session, which consisted of two parts:

1. Learning agenda - the founder of the community garden gave a lecture about rainwater catchment and aquifer recharge.

2. Percolation test - a homespun version of how one can measure the rate at which water moves through soil to showcase the percolation within dune sand/soil.

I regret not bringing a notebook with me, but will try to recap the event to the best of my ability while providing links to resources that can go into more detail about what I learned. 

Not a big fan of reading blog posts? I do a quick Tik Tok recap here.

Part 1: Learning Agenda - San Francisco Aquifers, Groundwater Usages, Diversion Alternatives

With the amount of Guinness I’d imbibed the day prior, I was happy that some of what we talked about at the beginning of this event had been briefly covered at the last Surfrider San Francisco chapter meeting. (Psst! The next Surfrider San Francisco Chapter Meeting is April 2nd, if you’re interested in joining).

This made it a lot easier for me to follow along and allowed us, as a group, to take a deeper dive into the subjects that were covered.

In the shade of that large tree, we sat in our chairs and were reminded that where we actually sat was on top of an aquifer.

What is an aquifer

An aquifer is an underground area made up of porous matter (sand, soil, sediment, etc.) where water has seeped into the ground and is stored in the spaces between that porous matter. 

Imagine filling a sealed planter with a few layers of pebbles and then topping those pebbles with soil. If you were to pour water over that soil, it would slowly trickle down to fill in the space between the pebbles below. The section of the planter where the water has seeped in between the pebbles is what would be the aquifer.

What is groundwater 

Groundwater is the water that is found in the aquifers discussed above. Rainwater that has soaked into the ground and dispersed between the porous materials of the aquifer. 

San Francisco’s Westside Basin

Here in San Francisco, we sit over a series of aquifers that stretch 45 square miles from Golden Gate Park through San Bruno (SFPUC Groundwater). This is called the Westside Basin. 

Why we should care about groundwater

Most of us are familiar with the Hetch Hetchy. Ask any San Francisco resident if it’s “safe to drink the water” and we all reply, “oh, yes! It’s the best tap water. We have the Hetch Hetchy.”

The reservoir is known for being a great water resource. 

However…  this isn’t the only place being sourced for our drinking water. 

San Francisco’s drinking water comes from a variety of sources including groundwater (SFPUC Water System).

So, what’s the issue?
It’s not that groundwater is unsafe to drink.

The groundwater that’s in our drinking water has been treated and blended to be safe for drinking.

According to the SFPUC, the goal is to blend four million gallons of treated groundwater with regional water supplies a day.

If this is the case, wouldn’t it be useful to know that the groundwater supply is being replenished?

The challenge then becomes how do we ensure that our aquifers are being recharged?

Rainwater Catchment 

If you look at most gutter systems hooked up to houses and buildings in your neighborhood, you may notice that you can’t actually see where rainwater is going. This is because it’s redirected into our sewage system. 

Even if rainwater was not redirected to our sewage system, there aren’t many places that it can penetrate to reach the land below the layers of asphalt and cement in our city.  

Not only are the recent storms causing issues with our sewage system (I shared more about that here), but this is also limiting the recharge of our aquifers which are being tapped more frequently.

SFPUC’s Green Infrastructure Project

The good news is that others are aware of the issues we’re facing (both our aging sewer system as well as the need to replenish groundwater). The SFPUC recently completed a greenway project running twelve blocks on Sunset Boulevard from Irving to Ulloa. This greenway consists of thirty rain gardens that utilize native plants and bioswales (landscape depressions designed to capture stormwater) among other things, while also providing educational materials along the greenway.

Part 2: The Percolation Test - How many gallons per minute can our native dune soil percolate?

There was a lot more discussed during the learning session that I won’t cover here. I highly recommend attending future sessions or volunteering for upcoming ocean-friendly garden workshops.  

Now, however, I want to get to the percolation test!

We learned how to measure the percolation rate of the natural dune sand (what the Outer Sunset district is built on top of) and what sits just below the top layer of soil in the Far Out West Community Garden.

The first step to measuring this percolation rate was to dig a hole.

We took turns digging until we hit the sandy soil of the dunes below. We kept digging until we had a hole just about a foot wide and a few feet deep. 

The founder of the community garden then brought over a large hollow pipe that had been drilled with holes on both sides. He wrapped it with nylon filter fabric to stop any water from rushing out.

He placed the pipe into the hole and we refilled it with the dune sand that we’d dug out.

Saturating the ground & filling the pipe with water

He explained that normally, before conducting this test, the ground would be saturated ahead of time. However, for this example, we used the garden hose to saturate the area around the tube as much as possible before attempting to fill it with water. 

Which brings me to the next step.

Once the ground was as saturated as it could be, we attempted to fill the pipe with water. Because the dune sand is so porous and we had not saturated the area before the test it was challenging to get water to flow fast enough from the hose to fill the tube. 

After a bit of a wait, the pipe had been filled almost to the top.

At this point, we placed another, much skinnier tube inside. The bottom of this tube had a floating device at one end (the bottom end) that had been made from recycled surfboard foam. This foam had been cut to be almost the same diameter as the larger pipe. We placed this smaller tube with its floating device inside the larger.

Along one side of the narrower pipe were inches were denoted in black Sharpie marker.

One person from our group used a stopwatch on their phone to time out ten seconds. Another group member carefully watched as the smaller tube disappeared inside the larger one. They announced the number of inches we saw disappear as the smaller tube sank down with the water that dispersed through the sandy dune soil

Multiplying this by six, we calculated the number of inches per minute.

Though we’d want to hire a professional geologist if conducting this test for a commercial or residential project, this homespun version allowed us to see how efficient dune sand is at percolating water. 

Why does the percolation rate matter?

Materials with a higher percolation rate can be beneficial for areas that have a lot of storms or high amounts of rainwater.

With the ability to soak up and disperse large amounts of water, areas with this type of porous matter (sands, gravel, stones) can help avoid overflow. This is especially important now that our sewage and water treatment system has become overwhelmed during big storms.

Rainwater Catchment Systems

After our percolation test, we looked at the rainwater catchment system that’s set up at the Far Out West Community Garden. 

This catchment system helps to disperse rainwater from the roof into that efficient percolating dune sand underground. We saw how the pipes connected to remove any big sediment and then how the water was fed through the system out to where large tubes underground allowed it to seep into that sandy soil below.

The End of the Day

The day was complete after the percolation test and the quick overview of the rain catchment set-up at the garden. 

After that, it was time to attempt my jog home up through the avenues, along JFK in Golden Gate Park, and through the Panhandle.

Home at last, after a full weekend, I felt happy to know so much more about ocean-friendly gardening, San Francisco’s water sources, and rainwater catchment systems.